【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;
    }
};