◻️1512. Number of Good Pairs (easy)

Given an array of integers nums, return the number of good pairs.

A pair (i, j) is called good if nums[i] == nums[j] and i < j.

Example 1:

Input: nums = [1,2,3,1,1,3]
Output: 4
Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

Example 2:

Input: nums = [1,1,1,1]
Output: 6
Explanation: Each pair in the array are good.

Example 3:

Input: nums = [1,2,3]
Output: 0

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Solutions

Optimised

Using Frequency Table

Runtime 0 ms Memory 9.09 MB

// Solution from https://algo.monster/liteproblems/1512
// T.C -> O(n)
// Using Hashmap (frequency table)
// For every element x, we add freqTable[x] to ans. Why? Because if freqTable[x] is the number of times we've seen x so far, then there are freqTable[x] ways to form a 'good pair' with x being the second element.

class Solution {
public:
    int numIdenticalPairs(vector<int>& nums) {
        int pairCount=0;
        vector<int> freqTable (101, 0);
        for(auto num: nums){
            pairCount += freqTable[num];
            freqTable[num]++;
        }
        return pairCount;
    }
};