Today I have done leetcode's November LeetCoding Challenge with cpp.

November LeetCoding Challenge 13

Description

Count the Number of Fair Pairs

Given a 0-indexed integer array nums of size n and two integers lower and upper, return the number of fair pairs.

A pair (i, j) is fair if:

• 0 <= i < j < n, and
• lower <= nums[i] + nums[j] <= upper

Example 1:

Input: nums = [0,1,7,4,4,5], lower = 3, upper = 6
Output: 6
Explanation: There are 6 fair pairs: (0,3), (0,4), (0,5), (1,3), (1,4), and (1,5).

Example 2:

Input: nums = [1,7,9,2,5], lower = 11, upper = 11
Output: 1
Explanation: There is a single fair pair: (2,3).

Constraints:

• 1 <= nums.length <= 105
• nums.length == n
• -109 <= nums[i] <= 109
• -109 <= lower <= upper <= 109

Solution

class Solution {
public:
  long long countFairPairs(vector<int>& nums, int lower, int upper) {
    sort(nums.begin(), nums.end());
    long long answer = 0;
    map<int, int> count;
    for(auto n : nums) {
      count[n] += 1;
      auto eit = upper_bound(nums.begin(), nums.end(), upper - n);
      if(eit == nums.begin()) continue;
      auto bit = lower_bound(nums.begin(), nums.end(), lower - n);
      answer += eit - bit;
    }
    for(const auto &[n, c] : count) {
      if(2 * n >= lower && 2 * n <= upper) answer -= c;
    }
    return answer / 2;
  }
};