Formatted question description: https://leetcode.ca/all/1512.html

1512. Number of Good Pairs (Easy)

Given an array of integers nums.

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

Return the number of good pairs.

 

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

Related Topics:
Array, Hash Table, Math

Solution 1. Brute force

// OJ: https://leetcode.com/problems/number-of-good-pairs/

// Time: O(N^2)
// Space: O(1)
class Solution {
public:
    int numIdenticalPairs(vector<int>& nums) {
        int ans = 0;
        for (int i = 0; i < nums.size(); ++i) {
            for (int j = i + 1; j < nums.size(); ++j) ans += nums[i] == nums[j];
        }
        return ans;
    }
};

Solution 2. Count

// OJ: https://leetcode.com/problems/number-of-good-pairs/

// Time: O(N)
// Space: O(N)
class Solution {
public:
    int numIdenticalPairs(vector<int>& nums) {
        unordered_map<int, int> m;
        int ans = 0;
        for (int n : nums) ans += m[n]++;
        return ans;
    }
};

Java

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

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