NumberOfGoodParisSort.java

public int numIdenticalPairsWithSort(int[] nums) {
    if (nums == null || nums.length == 0) {
        return 0;
    }

    Arrays.sort(nums);

    int goodPairsCount = 0;
    int start = 0;
    int end = 0;

    while (end < nums.length) {
        if (nums[end] == nums[start]) {
            end++;
            continue;
        }
        // if you have n occurrences of a number, the good pair is n*(n-1)/2, Cn^2
        int delta = end - start;
        if (delta > 1) {
            goodPairsCount += delta * (delta - 1) / 2;
        }
        start = end;
    }
    // handle the case 1, 2, 3, 3, 3
    if (start != nums.length - 1) {
        goodPairsCount += (end - start) * (end - start - 1) / 2;
    }
    return goodPairsCount;
}