# 2563. Count the Number of Fair Pairs
###### tags: `Leetcode` `Medium` `Two Pointers`
Link: https://leetcode.com/problems/count-the-number-of-fair-pairs/description/
## 思路

Number of pairs with sum >= lower and <= upper
= (Number of pairs with sum <= upper) - (Number of pairs with sum < lower)

双指针找出所有和小于某个值的pair数
## Code

class Solution {
    public long countFairPairs(int[] nums, int lower, int upper) {
        Arrays.sort(nums);
        return countLess(nums, upper)-countLess(nums, lower-1);
    }
    public long countLess(int[] nums, int bound){
        long res = 0;
        int i = 0, j = nums.length-1;
        while(i<j){
            while(i<j && nums[i]+nums[j]>bound) j--;
            res += j-i;
            i++;
        }
        return res;
    }
}

class Solution:
    def countFairPairs(self, nums: List[int], lower: int, upper: int) -> int:
        nums.sort()
        
        def countLessThan(nums, target):
            ans = 0
            i, j = 0, len(nums)-1
            while i<j:
                while i<j and nums[i]+nums[j]>target: j-=1
                ans += j-i
                i += 1
            return ans

        return countLessThan(nums, upper)-countLessThan(nums, lower-1)