[leetcode] 16. 3Sum Closest 解题报告

题目链接：https://leetcode.com/problems/3sum-closest/

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).

思路：和3sum思路差不多，如果暴力搜索，其时间复杂度将会是O（n^3）。我们可以先挑出一个数来，然后问题将同样退化为2sum问题，即找出两个数等于另外一个数，其时间复杂度将会降为O（n^2）。首先将数组排序，然后使用左右指针，寻找最靠近target的数值。

代码如下：

class Solution {public:    int threeSumClosest(vector<int>& nums, int target) {        sort(nums.begin(), nums.end());        int len = nums.size(), ans = nums[0] + nums[1] + nums[2];        for(int i = 0; i < len; i++)        {            int left = i+1, right = len-1;            while(left < right)            {                int val = nums[i] + nums[left] + nums[right];                if(labs(target - ans) > labs(target - val)) ans = val;                val<target?left++:right--;            }        }        return ans;    }};