LeetCode OJ

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).

解题思路：

枚举第一个数，另外两个数：一个在第一个数后边一个开始，一个从末尾开始，和4Sum类似调整。复杂度：O(n^2)。

代码：

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
       int ans = 0;
       bool findans = false;
       sort(num.begin(), num.end());
       for (int first = 0; first < num.size(); first++){
           for (int second = first + 1, third = num.size() - 1; second < third; ){
               int tmp_sum = num[first] + num[second] + num[third];
               if (tmp_sum < target){
                   second ++;
               }
               if (tmp_sum > target){
                   third --;
               }
               if (tmp_sum == target){
                   return tmp_sum;
               }
               
               if (!findans || (abs(tmp_sum - target) < abs(ans - target))){
                   ans = tmp_sum; 
                   findans = true;
               }
           }
       }
       return ans;
    }
};