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.

思路：这题，其实根本不用管 i < j,sort 之后，同向双指针查找，只要 A[i] != A[j]就可以，小的放i，大的放j即可；O(NlogN)

class Solution {
    public int twoSumLessThanK(int[] A, int K) {
        Arrays.sort(A);
        int i = 0; int j = A.length - 1;
        int maxsum = -1;
        while(i < j) {
            int sum = A[i] + A[j];
            if(sum < K) {
                maxsum = Math.max(maxsum, sum);
                i++;
            } else {
                //sum >= K;
                j--;
            }
        }
        return maxsum;
    }
}