Number of Good Pairs LeetCode

 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


Brute Force:


class Solution {
    public int numIdenticalPairs(int[] nums) {
     int count=0;
     for(int i=0;i<nums.length;i++)
     {
        for(int j=1;j<nums.length;j++)
        { if(nums[i]==nums[j] && i<j)
        {
            count++;
        }

        }
       
     }  
     return count;
    }
}


Optimal Approach:

class Solution {
    public int numIdenticalPairs(int[] nums) {
         // This map will store how many times each number appears
        HashMap<Integer, Integer> map = new HashMap<>();

        int count = 0;

        // Loop through every number in the array
        for (int num : nums) {
            // If we have already seen this number
            if (map.containsKey(num)) {
                // Add the number of times we've seen it to the count
                count += map.get(num);

                // Update the count of this number in the map
                map.put(num, map.get(num) + 1);
            } else {
                // First time seeing this number, so store it as 1
                map.put(num, 1);
            }
        }

        // Return the total number of good pairs
        return count;
    }
}


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