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.

 Example 1:

Input: nums = [4,1,3,3]
Output: 5
Explanation: The pair (0, 1) is a bad pair since 1 - 0 != 1 - 4.
The pair (0, 2) is a bad pair since 2 - 0 != 3 - 4, 2 != -1.
The pair (0, 3) is a bad pair since 3 - 0 != 3 - 4, 3 != -1.
The pair (1, 2) is a bad pair since 2 - 1 != 3 - 1, 1 != 2.
The pair (2, 3) is a bad pair since 3 - 2 != 3 - 3, 1 != 0.
There are a total of 5 bad pairs, so we return 5.

Example 2:

Input: nums = [1,2,3,4,5]
Output: 0
Explanation: There are no bad pairs.

 Constraints:

• 1 <= nums.length <= 105
• 1 <= nums[i] <= 109

JAVA

• class Solution {
      public long countBadPairs(int[] nums) {
          Map<Integer, Integer> cnt = new HashMap<>();
          long ans = 0;
          for (int i = 0; i < nums.length; ++i) {
              int x = i - nums[i];
              ans += i - cnt.getOrDefault(x, 0);
              cnt.merge(x, 1, Integer::sum);
          }
          return ans;
      }
  }

C++

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

PYTHON

• from collections import defaultdict
  
  class Solution:
      def countBadPairs(self, nums):
          cnt = defaultdict(int)
          ans = 0
          for i, num in enumerate(nums):
              x = i - num
              ans += i - cnt[x]
              cnt[x] += 1
          return ans