leetcode No16. 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).

Algorithm:

先排序，用一层循环加两个指针j,k从两边（i+1,k=nums.size()）往中间扫，遇到最接近的就更新res，如果比target大则k--,反之j++。

Accepted Code：

class Solution {public:    int threeSumClosest(vector<int>& nums, int target) {        if(nums.size()==3)return nums[0]+nums[1]+nums[2];        sort(nums.begin(),nums.end());        int res = nums[0]+nums[1]+nums[2];        int sum = 0;        for(int i=0;i<nums.size();i++)        {            int j = i+1;            int k = nums.size()-1;            while(j<k)            {                sum = nums[i]+nums[j]+nums[k];                if(sum == target)                    return sum;                if(abs(sum-target) < abs(res-target))                {                    res = sum;                }                if(sum > target)                    k--;                else                    j++;            }        }        return res;    }};