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).

意思：他的大概意思是说给定一个数组和一个target，寻找出3个相加的值最接近target的三个数。

思路：其实和之前的3Sum差不多，只是要求的值是最接近的，我们可以不断地比较值的绝对值最接近于target的那个数就是我们要求的

代码：

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        int closetarget =nums[0]+nums[1]+nums[2];
        if(closetarget==target||nums.size()==3) return closetarget;
        sort(nums.begin(),nums.end());
        for(int i=0;i<nums.size();i++){
            int front =i+1;
            int back=nums.size()-1;
            while(front<back){
                int sum =nums[i]+nums[front]+nums[back];
                if(abs(closetarget-target)>abs(sum-target)){
                    closetarget =sum;
                }
                if(sum<target) front++;
                else if(sum>target) back--;
                else return sum;
            }
        }
        return closetarget;
    }
};