leetcode-1512-easy

Number of Good Pairs
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

思路一：map 存储，最后遍历 map 统计

    public int numIdenticalPairs(int[] nums) {
        Map<Integer, Integer> map = new HashMap<>();

        for (int num : nums) {
            map.compute(num, (k, v) -> v == null ? 1 : v + 1);
        }

        int ans = 0;

        for (Map.Entry<Integer, Integer> e : map.entrySet()) {
            if (e.getValue() >= 2) {
                int x = e.getValue() - 1;
                ans = ans + ((1 + x) * x / 2);
            }
        }

        return ans;
    }