Count the Number of Fair Pairs | Leetcode 2563

class Solution {
    #define ll long long
public:
    long long countFairPairs(vector<int>& nums, int lower, int upper) {
        int n=nums.size();
        sort(nums.begin(),nums.end());

        ll ans=0;
        for(int i=0;i<n-1;++i){
            int lb = lower_bound(nums.begin()+i+1,nums.end(),lower-nums[i])-nums.begin();
            int ub = upper_bound(nums.begin()+i+1,nums.end(),upper-nums[i])-nums.begin();
            ans += (ub - lb);
        }
        return ans;
    }
};

/*
//JAVA
import java.util.*;

class Solution {
    public long countFairPairs(int[] nums, int lower, int upper) {
        int n = nums.length;
        Arrays.sort(nums);
        
        long ans = 0;
        for (int i = 0; i < n - 1; ++i) {
            int lb = lowerBound(nums, i + 1, n, lower - nums[i]);
            int ub = upperBound(nums, i + 1, n, upper - nums[i]);
            ans += (ub - lb);
        }
        return ans;
    }
    
    private int lowerBound(int[] nums, int start, int end, int target) {
        while (start < end) {
            int mid = start + (end - start) / 2;
            if (nums[mid] < target) start = mid + 1;
            else end = mid;
        }
        return start;
    }

    private int upperBound(int[] nums, int start, int end, int target) {
        while (start < end) {
            int mid = start + (end - start) / 2;
            if (nums[mid] <= target) start = mid + 1;
            else end = mid;
        }
        return start;
    }
}

#Python
from bisect import bisect_left, bisect_right

class Solution:
    def countFairPairs(self, nums, lower, upper):
        nums.sort()
        ans = 0
        n = len(nums)
        
        for i in range(n - 1):
            lb = bisect_left(nums, lower - nums[i], i + 1)
            ub = bisect_right(nums, upper - nums[i], i + 1)
            ans += (ub - lb)
            
        return ans

*/