Question：

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).

Anwser 1：

O（n^2）

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int len = num.size();
        assert(len >= 3);
        
        vector<int> num2;
        num2.assign(num.begin(), num.end());
        
        sort(num2.begin(), num2.end());
        
        int sum = num2[0] + num2[1] + num2[2];
        int diff = abs(sum - target);
        int ret = sum;
        
        for(int i = 0; i < len - 2; i++){
            int l = i + 1;
            int r = len - 1;
            
            while(l < r){
                sum = num2[i] + num2[l] + num2[r];
                
                if(sum == target) {
                    return sum;
                } else if (sum < target){
                    l++;
                } else {
                    r--;
                }
                
                if(abs(sum - target) < diff){       // more close, refresh diff and ret
                    diff = abs(sum - target);
                    ret = sum;
                }
            }
        }
        return ret;
    }
};

参考推荐：

3Sum

4Sum

数组中最大和的子数组