Number of Good Pairs

1using System;
2using System.Collections.Generic;
3
4public class Solution {
5    // time O(n)
6    // space O(n)
7    public int NumIdenticalPairs(int[] nums) {
8        int count = 0;
9        Dictionary<int, int> dict = new Dictionary<int, int>();
10
11        for (int i = 0; i < nums.Length; i++)
12        {
13            if (dict.ContainsKey(nums[i]))
14            {
15                count += dict[nums[i]];
16                dict[nums[i]]++;
17            }
18            else
19            {
20                dict[nums[i]] = 1;
21            }
22        }
23
24        return count;
25    }
26
27    // time O(n^2)
28    // space O(1)
29    // public int NumIdenticalPairs(int[] nums) {
30    //     int count = 0;
31
32    //     for (int i = 0; i < nums.Length; i++)
33    //     {
34    //         for (int j = i + 1; j < nums.Length; j++)
35    //         {
36    //             if (nums[i] == nums[j])
37    //             {
38    //                 count++;
39    //             }
40    //         }
41    //     }
42
43    //     return count;
44    // }
45
46    void Test()
47    {
48        int[] nums1 = {1,2,3,1,1,3};
49        int actual1 = NumIdenticalPairs(nums1);
50        if (actual1 != 4)
51        {
52            throw new Exception("Expected 4, but got " + actual1);
53        }
54
55        int[] nums2 = {1,1,1,1};
56        int actual2 = NumIdenticalPairs(nums2);
57        if (actual2 != 6)
58        {
59            throw new Exception("Expected 6, but got " + actual2);
60        }
61
62        int[] nums3 = {1,2,3};
63        int actual3 = NumIdenticalPairs(nums3);
64        if (actual3 != 0)
65        {
66            throw new Exception("Expected 0, but got " + actual3);
67        }
68
69        int[] nums4 = {6,5,1,5,7,7,9,1,5,7,1,6,10,9,7,4,1,8,7,1,1,8,6,4,7,4,10,5,3,9,10,1,9,5,5,4,1,7,4,2,9,2,6,6,4,2,10,3,5,3,6,4,7,4,6,4,4,6,3,4,10,1,10,6,10,4,9,6,6,4,8,6,9,5,4};
70        int actual4 = NumIdenticalPairs(nums4);
71        if (actual4 != 303)
72        {
73            throw new Exception("Expected 303, but got " + actual4);
74        }
75    }
76
77    public static void Main(string[] args)
78    {
79        new Solution().Test();
80    }
81}