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;
    }
}

一开始我是拿brute force来求解的。
后来突然想起来了可以排序。然后这个做法就自然而然出来了。

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

还有种做法就是用binary search,
定住 i,j, binary search k so that nums[k] < target – nums[i] – nums[j]

find the biggest k

注意这里写binary search, mid = (begin + end + 1) / 2;
以防止进入死循环。

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