LeetCode 1099. Two Sum Less Than K

原题链接在这里：https://leetcode.com/problems/two-sum-less-than-k/

题目：

Given an array A of integers and integer K, return the maximum S such that there exists i < j with A[i] + A[j] = S and S < K. If no i, jexist satisfying this equation, return -1.

Example 1:

Input: A = [34,23,1,24,75,33,54,8], K = 60
Output: 58
Explanation: 
We can use 34 and 24 to sum 58 which is less than 60.

Example 2:

Input: A = [10,20,30], K = 15
Output: -1
Explanation: 
In this case it's not possible to get a pair sum less that 15.

Note:

1. 1 <= A.length <= 100
2. 1 <= A[i] <= 1000
3. 1 <= K <= 2000

题解：

Sort the array. Use two pointers pointing at head and tail. Calculate the sum.

If sum < K, update res and l++ to increase sum.

Otherwise, r-- to decrease sum.

Time Complexity: O(nlogn). n = A.length.

Space: O(1).

AC Java:

class Solution {
    public int twoSumLessThanK(int[] A, int K) {
        int res = -1;
        if(A == null || A.length == 0){
            return res;
        }
        
        Arrays.sort(A);
        int l = 0;
        int r  = A.length-1;
        while(l<r){
            int sum = A[l] + A[r];
            if(sum < K){
                res = Math.max(res, sum);
                l++;
            }else{
                r--;
            }
        }
        
        return res;
    }
}

类似Subarray Product Less Than K, 3Sum Smaller.