【leetcode】3Sum Closest（middle）

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).

 

思路：

先排序，然后固定一个边界，另外两个边界收缩。

class Solution {

public:

    int threeSumClosest(vector<int> &num, int target) {

        int l, r, m;

        int ans = num[0] + num[1] + num[2];

        sort(num.begin(), num.end()); //数字从小到大排序

        for(l = 0; l < num.size() - 2; l++) //固定左边界

        {

            m = l + 1;

            r = num.size() - 1;

            while(m < r) //收缩中间和右边界

            {

                int tmp = num[l] + num[m] + num[r];

                ans = (abs(ans - target) > abs(tmp - target)) ? tmp : ans;

                if(tmp < target)

                {

                    (num[m] > num[r]) ? r-- : m++; //比目标小 则把小的数字变大

                }

                else if(tmp > target)

                {

                    (num[m] < num[r]) ? r-- : m++; //比目标大 则把大的数字变小

                }

                else

                {

                    return tmp; //已经等于target就直接返回

                }

            }

        }

        return ans;

    }

};