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//Approach-3 (Using hashmap)
//T.C : O(n)
//S.C : O(n)
class Solution {
    public long countBadPairs(int[] nums) {
        int n = nums.length;
        long result = 0;
        HashMap<Integer, Integer> mp = new HashMap<>();
        
        for (int i = 0; i < n; i++) {
            int diff = nums[i] - i;
            int totalPairsTillIndex = i;
            int goodPairs = mp.getOrDefault(diff, 0);
            
            result += (totalPairsTillIndex - goodPairs);
            
            mp.put(diff, goodPairs + 1);
        }
        
        return result;
    }
}