Number of Good Pairs | Leetcode Problem No. 1512 | Array

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

Constraints:

• 1 <= nums.length <= 100

• 1 <= nums[i] <= 100

Intuition:

• If there are n elements, then there will be n * (n-1)/2 pairs can be formed

public class NumberOfGoodPairs {

    /**
     * Two traversal Approach
     *
     * @param nums
     * @return
     */
   public int numIdenticalPairs_twoTraversal(int[] nums) {
        var map = new HashMap<Integer, Integer>();
        var count = 0;

        for(int i=0; i<nums.length; i++) {
            map.put(nums[i], map.getOrDefault(nums[i], 0)+1);
        }

        for(Map.Entry<Integer, Integer> itr : map.entrySet()){
            int n = itr.getValue();
            count += (n*(n-1))/2;
        }
        return count;
    }

    /**
     * Single traversal
     * @param nums
     * @return
     */
    public int numIdenticalPairs_singleTraversal(int[] nums) {
        var map = new HashMap<Integer, Integer>();
        var count = 0;

        for(int i=0; i<nums.length; i++) {
            int currElementCount = map.getOrDefault(nums[i], 0);
            count += currElementCount;
            map.put(nums[i], currElementCount+1);
        }
        return count;
    }
}