My code:

public class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        int ret = 0;
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 2; i++) {
            int begin = i + 1;
            int end = nums.length - 1;
            int tar = target - nums[i];
            while (begin < end) {
                int sum = nums[begin] + nums[end];
                if (sum < tar) {
                    ret += end - begin;
                    begin++;
                }
                else {
                    end--;                    
                }
            }
        }
        
        return ret;
    }
}

一開始我是拿brute force來求解的。
后來突然想起來了可以排序。然后這個做法就自然而然出來了。

reference:
https://leetcode.com/articles/3sum-smaller/

還有種做法就是用binary search,
定住 i,j, binary search k so that nums[k] < target - nums[i] - nums[j]

find the biggest k

注意這里寫binary search, mid = (begin + end + 1) / 2;
以防止進入死循環。

Anyway, Good luck, Richardo! -- 09/05/2016