Solving Leetcode Problems

Learn how to solve the Leetcode problem of id 283, whose title is Move Zeroes, using the Java programming language.

https://leetcode.com/problems/move-zeroes

The Data Structures and Algorithms (DSA) lesson uses a two-pointer approach to solving the question using Java.

Traversing the array you can copy all non-zero elements to its place at the left-hand side of the array. Then later you can fill all the remaining values at the right-hand side with a zero.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Move Zeroes - Leetcode 283 - Java

Learn how to solve the Leetcode problem of id 1480, whose title is Running Sum of 1d Array, using the Java programming language.

https://leetcode.com/problems/running-sum-of-1d-array

The Data Structures and Algorithms (DSA) lesson uses a running sum with a for-loop approach to solving the question using Java.

Traversing the array, you can keep track of the sum so far in an integer variable. Then you can use the array itself and replace its current value with the running sum value.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Running Sum of 1d Array - Leetcode 1480 - Java

Learn how to solve the Leetcode problem of id 1732, whose title is Find the Highest Altitude, using the Java programming language.

https://leetcode.com/problems/find-the-highest-altitude

The Data Structures and Algorithms (DSA) lesson uses a running sum with a for-loop approach to solving the question using Java.

Traversing the array, you can keep track of the sum so far in an integer variable. The absolute altitude at every iteration is the value of the sum. You also keep track of the highest number seen so far, which is determined by comparison with the current value of the sum. 

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Find the Highest Altitude - Leetcode 1732 - Java

Learn how to solve the Leetcode problem of id 75, whose title is Sort Colors, using the Java programming language.

https://leetcode.com/problems/sort-colors

The Data Structures and Algorithms (DSA) lesson uses a three-pointer approach to solving the question using Java.

Since the zeroes need to be placed at the left-hand side of the resulting array, we use a pointer initialized to 0 to keep track of the index for the zeroes ("Zero Pointer"). Similarly, you create another pointer initialized to length of array minus one to keep track of the index for the twos ("Two Pointer").

With a separate index pointer ("Position Pointer") to traverse the array, you look at what integer you are faced with. It will be either 0, 1, or 2.

If it's 0, swap the element at the Zero Pointer with the current position and fill in with 0 at the Zero Pointer; increment both Zero Pointer and Position Pointer.

If it's 2, swap the element at the Two Pointer with the current position, filling with 2 the value at Two Pointer; decrement the Zero Pointer.

If you find one, simply increment the Position Pointer.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Sort Colors - Leetcode 75 - Java

Learn how to solve the Leetcode problem of id 1920, whose title is Build Array from Permutation, using the Java programming language.

https://leetcode.com/problems/build-array-from-permutation

The Data Structures and Algorithms (DSA) lesson uses a loop approach to solving the question.

A new array of integers is created with the same length as the given one.

Then a for loop is used to traverse the elements of the array, from the first to the last, using an index variable.

You need to know how to access an array element by its index. In Java primitive array elements can be accessed using square brackets.

You assign the nums of the nums at position i to the corresponding element of the new array.

The time complexity for the solution is O(n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Build Array from Permutation - Leetcode 1920 - Java

Learn how to solve the Leetcode problem of id 3110, whose title is Score of a String, using the Java programming language.

https://leetcode.com/problems/score-of-a-string

The Data Structures and Algorithms (DSA) lesson uses a loop approach with running sum to solve the question.

Keep track of the score in an integer variable, initializing it to zero.

Then a for loop is used to traverse the elements of the array, from the first to the second before last, using an index variable.

The difference in the integer representation between two characters can be obtained by their subtraction in Java. Alternatively, you can use the codePointAt method to retrieve the integer value for each individual character.

The absolute value of the difference is added to the running sum of the score.

The absolute value is negative the value if it is negative (making it positive), or as is if it is zero or positive.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Score of a String - Leetcode 3110 - Java

Learn how to solve the Leetcode problem of id 344, whose title is Reverse String, using the Java programming language.

https://leetcode.com/problems/reverse-string

The Data Structures and Algorithms (DSA) lesson uses a loop approach with pointers to first and last element solve the question.

The first element of the array has to be swapped with the last element.

The second element of the array has to be swapped with the second to last element.

The swapping repeats with the left and right pointers converging to the element in the middle of the array.

Knowing that approach, you can iterate over the elements in the first half of the array.

The index of the right-hand side element that needs to be swapped is given by the formula n - 1 - i.

So, you always swap the i-th element with the (n - 1 - i)-th element.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Reverse String - Leetcode 344 - Java

Learn how to solve the Leetcode problem of id 242, whose title is Valid Anagram, using the Java programming language.

https://leetcode.com/problems/valid-anagram

The Data Structures and Algorithms (DSA) lesson uses an array of counts to keep track of the discrepancy in the frequency of the characters.

Right off the bat, if the length of the two strings is not the same, they are not a valid anagram.

If the strings have the same length, we have to check if the frequency of the characters in each string matches.

When a character is seen on the first string s, the count for that specific character is incremented.

When a character is seen on the second string t, the count for the specific character is decremented.

That way, when a character is both seen in the first string and the second string, the count for that should be zero because they will cancel out.

So, the problem in the end is about going through the array of counts to see if any of them is not zero, meaning there is a discrepancy. 

If all the counts end up being 0 that means it is a valid anagram since each repeated character canceled out in one string to another.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Valid Anagram - Leetcode 242 - Java

Learn how to solve the Leetcode problem of id 2011, whose title is Final Value of Variable After Performing Operations, using the Java programming language.

https://leetcode.com/problems/final-value-of-variable-after-performing-operations

The Data Structures and Algorithms (DSA) lesson uses an iterative approach to solve the problem.

Going through the elements of the array of operations, you can check and compare the strings against the specified directives that either increment or decrement the value of x.

An alternative solution is also provided at the end, due to the constraints for the input always having either a plus or minus character in the middle of the string, as the second element.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Final Value of Variable After Performing Operations - Leetcode 2011 - Java

Learn how to solve the Leetcode problem of id 136, whose title is Single Number, using the Java programming language.

https://leetcode.com/problems/single-number

The Data Structures and Algorithms (DSA) lesson uses a bit manipulation approach to solve the problem.

The exclusive-OR (XOR) between two binary numbers is zero if they are the same and one if they are different.

Knowing that, you get the following properties: a XOR a is 0; a XOR 0 is a.

That is, a binary number XOR itself will yield zero.

A binary number XOR zero will yield the number itself.

In addition, knowing that properties such as Commutative and Associative allow the rearranging of terms as you write the elements in the array XOR'ed together, you conclude the following:

Duplicate numbers will cancel each other out and the single number will remain.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Single Number - Leetcode 136 - Java

Learn how to solve the Leetcode problem of id 412, whose title is Fizz Buzz, using the Java programming language.

https://leetcode.com/problems/fizz-buzz

The Data Structures and Algorithms (DSA) lesson uses an iterative approach to solve the problem.

You can check whether a number is divisible by 3 if you use the modulo or remainder operator, usually represented by a percent sign.

A number will be divisible if there is no remainder. That is, the result is 0.

Similarly, you can use the number followed by percent sign followed by five. The result will be zero if divisible by 5.

Alternatively, you can also directly check a number is divisible by the product of 3 and 5, that is 15. Since mathematically a number divisible by 3 and by 5 must be divisible by 15.

The problem uses an if-else-if-else statement to take care of the cases.

First the case for divisible by both 3 and 5. Add FizzBuzz string to the list.

Then the case where only divisible by 3. Add Fizz string to the list.

Then the case where only divisible by 5. Add Buzz string to the list.

Then the case where none of the conditions above were satisfied. In that case, simply add the iterating index value as a string to the list.

The time complexity for the solution is O(n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Fizz Buzz - Leetcode 412 - Java

Learn how to solve the Leetcode problem of id 27, whose title is Remove Element, using the Java programming language.

https://leetcode.com/problems/remove-element

The Data Structures and Algorithms (DSA) lesson uses an iterative approach with a position pointer to solve the problem.

Initialize a pointer k that points to the first element in the array.

The pointer k will be the destination swap position of the element that doesn't match the given val, as you iterate through the array.

If the current value matches the val you want to remove, you simply ignore it and move on to the next iteration.

However, a value that does not match the value you want to remove would be copied to the position k.

By the end of the loop, all of the non-val elements would have been copied to the left-hand side of the array.

The right-hand side of the array would just be ignored, deemed garbage.

Since k increases everytime a non-matching value is seen, by the end of the loop k holds the count of elements of the array that are not equal to the remove value.

The time complexity for the solution is O(n) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Remove Element - Leetcode 27 - Java

Learn how to solve the Leetcode problem of id 2469, whose title is Convert the Temperature, using the Java programming language.

https://leetcode.com/problems/convert-the-temperature

The Data Structures and Algorithms (DSA) lesson uses a mathematical approach to solve the problem.

Simply compute the Kelvin and Fahrenheit values using the given formula and return them with an array of two elements.

The time complexity for the solution is O(1) and its space complexity is O(1).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Convert the Temperature - Leetcode 2469 - Java

Learn how to solve the Leetcode problem of id 1, whose title is Two Sum, using the Java programming language.

https://leetcode.com/problems/two-sum

The Data Structures and Algorithms (DSA) lesson uses a hash map approach to solve the problem.

Mathematically, the sum of a and b resulting in target can be expressed as one number being the target minus the other number.

So we can solve the problem by iterating over the integers and computing the difference between the target and the current number.

If the difference is a number that we have seen before, as recorded in a hash map, then we return the indices of the number seen before and the current index.

Otherwise, record that the current number was seen by placing a key-value pair of the number and its index in the array, respectively.

The time complexity for the solution is O(n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.

Two Sum - Leetcode 1 - Java

Learn how to solve the Leetcode problem of id 1, whose title is Two Sum, using the Java programming language.

https://leetcode.com/problems/two-sum

The Data Structures and Algorithms (DSA) lesson uses a hash map approach to solve the problem.

Mathematically, the sum of a and b resulting in target can be expressed as one number being the target minus the other number.

So we can solve the problem by iterating over the integers and computing the difference between the target and the current number.

If the difference is a number that we have seen before, as recorded in a hash map, then we return the indices of the number seen before and the current index.

Otherwise, record that the current number was seen by placing a key-value pair of the number and its index in the array, respectively.

The time complexity for the solution is O(n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Learn how to solve the Leetcode problem of id 1431, whose title is Kids With the Greatest Number of Candies, using the Java programming language.

https://leetcode.com/problems/kids-with-the-greatest-number-of-candies

The Data Structures and Algorithms (DSA) lesson uses an iterative approach to solve the problem.

First go through the array to find the highest value.

Then, do another pass and calculate whether the current element plus the extra number of candies is greater than or equal to the highest value found in the first pass.

The time complexity for the solution is O(n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.


Kids With the Greatest Number of Candies - Leetcode 1431 - Java

Learn how to solve the Leetcode problem of id 771, whose title is Jewels and Stones, using the Java programming language.

https://leetcode.com/problems/jewels-and-stones

The Data Structures and Algorithms (DSA) lesson uses an iterative approach with a hash set to solve the problem.

First go through the array of jewels to populate a hash set with its values.

Then, go through the array of stones. Add to the count if the current stone element is a member of the set of jewels.

The time complexity for the solution is O(m + n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.

Jewels and Stones - Leetcode 771 - Java

Learn how to solve the Leetcode problem of id 771, whose title is Jewels and Stones, using the Java programming language.

https://leetcode.com/problems/jewels-and-stones

The Data Structures and Algorithms (DSA) lesson uses an iterative approach with a hash set to solve the problem.

First go through the array of jewels to populate a hash set with its values.

Then, go through the array of stones. Add to the count if the current stone element is a member of the set of jewels.

The time complexity for the solution is O(m + n) and its space complexity is O(n).

DSA problems are sometimes asked during tech job interviews for positions such as Software Engineer, so you can use the challenge to practice that skill.
