3Sum Closest

題目：
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).

思路：
1、和前一個求三個數之和爲0相同，只是多加了一個差值的判斷；
代碼：

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        sort(num.begin(),num.end());
        int res=0;
        bool first=true;
        for(int i=0;i<num.size()-1;i++)
        {
            int j=i+1;
            int k=num.size()-1;

            while(j<k)
            {
                int sum=num[i]+num[j]+num[k];
                if(first)
                {
                    res=sum;
                    first=false;
                }
                else
                {
                    if(abs(sum-target)<abs(res-target))
                        res=sum;
                }
                if(res==target) return res;
                if(sum>target) k--;
                else j++;
            }
        }
        return res;
    }
};