[LeetCode] 3Sum Closest

3Sum Closest

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：

类似于3Sum。首先给数组排序，用两个变量分别记录当前最小距离和最近的和。代码如下，时间复杂度为O(n^2)

class Solution {public:    int threeSumClosest(vector<int> &num, int target) {        int len=num.size();        if(len<3){            return 0;        }        std::sort(num.begin(), num.end());        int closest=num[0]+num[1]+num[2];        int minDis = abs(closest-target);        for(int i=0; i<len; i++){            int start=i+1, end=len-1;            while(start<end){                int sum=num[i]+num[start]+num[end];                int dis=abs(sum-target);                if(dis<minDis){                    minDis=dis;                    closest=sum;                }                if(sum>target){                    end--;                }else{                    start++;                }            }        }        return closest;    }        int abs(int a){        return a>=0?a:-a;    }};