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).

从一个数组中找到三个数，使这三个数的和与target的值最接近，返回这三个数的和。

2.思路 

同上一题一样，固定一个数，另外两个数由两边向中间遍历，不断更新三个数的和，若刚好等于target，退出循环。

class Solution {public:    int threeSumClosest(vector<int>& nums,int target) {        int ans=nums[0]+nums[1]+nums[2];        int len=nums.size();        sort(nums.begin(),nums.end());        int i=0;        while(i<len-2)        {            int head=i+1,tail=len-1;            while(head<tail)            {                int sum=nums[i]+nums[head]+nums[tail];                if(abs(sum-target)<abs(ans-target))                    ans=sum;                if(sum == target)                {                    return target;break;                }                else if(sum > target)                    tail--;                else                    head++;             }             i++;        }        return ans;    }};