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).answer:
class Solution {public:    int threeSumClosest(vector<int>& nums, int target) {        int gap = INT_MAX;        int flag = 1;        int sum = 0;        sort(nums.begin(), nums.end());        for(int i = 0; i < nums.size(); i ++){            int start = i + 1;            int end = nums.size() - 1;            while(start < end){                sum = nums[i] + nums[start] + nums[end];                if(sum < target){                    start ++;                    if(target - sum < gap){                        gap = target - sum;                        flag = -1;                    }                }                else if(sum > target){                    end --;                    if(sum - target < gap){                        gap = sum - target;                        flag = 1;                    }                }                else{                    return target;                }            }        }        return target + flag * gap;    }};