1099. Two Sum Less Than K

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, j exist 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

// sort + two pointers
int twoSumLessThanK(vector<int>& A, int K) { // time: O(nlogn); space: O(1)
    sort(A.begin(), A.end());
    int n = A.size(), i = 0, j = n - 1, res = -1;
    while (i < j) {
        int sum = A[i] + A[j];
        if (sum < K) {
            res = max(res, sum);
            ++i;
        } else {
            --j;
        }
    }
    return res;
}