[leetcode 16] 3Sum Closest

1 题目

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

    For example, given array S = {-1 2 1 -4}, and target = 1.

    The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

2 思路

好吧，过了好几个月没刷了，这个题做的很蛋疼，花了4、5个小时。首先是题目意思理解错误，然后再理解对了的情况下，思路又错了。不得不看别人答案。

这道题原来最好的解决方法都是o(n^2)。

既然要求时间复杂度是o(n^2)，那么思路就好办了。遍历所有的3个组合，看哪个距target最近就ok了。采用的方法是一个头指针、一个尾指针，一个current指针，详情，看代码。

3 代码

    public int threeSumClosest(int[] nums, int target)
    {
        int sum = 0;
        if (nums.length <= 3)
        {
            for (int i : nums) {
                sum += i;
            }
            return sum;
        }
        Arrays.sort(nums);
        sum = nums[0]+nums[1]+nums[2];
        int add = sum;
        for (int i = 0; i < nums.length - 2; i++) {
            int head = i+1;
            int end = nums.length-1;
            while(head < end){
                add = nums[i] + nums[head] + nums[end];
                if (add < target ) {
                    head++;
                }else{
                    end--;
                }
                
                if (Math.abs(add-target) < Math.abs(sum-target)) {
                    sum = add;
                }
            }
        }
        return sum;
    }

今后，尽量，两天一题。