2364. Count Number of Bad Pairs

You are given a 0-indexed integer array nums. A pair of indices (i, j) is a bad pair if i < j and j - i != nums[j] - nums[i].

Return the total number of bad pairs in nums.

class Solution {
public:
    long long countBadPairs(vector<int>& nums) {
        long long res = 0;
        unordered_map<int, int> freq;
        for(int i = 0; i < nums.size(); i++) {
            res += i - freq[nums[i] - i];
            freq[nums[i] - i]++;
        }
        return res;
    }
};