【leetcode】3Sum Closest

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;    }};

參考推薦：

3Sum

4Sum

數組中最大和的子數組