LeetCode 3Sum Closest 最接近目标数的三个数和

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).

这道题和3Sum差不多，不过也有不一样的，主要是：

1 这里不用判断处理重复问题

2 要比较其中的三个数的和与目标数的差的大小。

class Solution {  
public:  
    int threeSumClosest(vector &num, int target)  
    {  
        switch (num.size())  
        {  
        case 0:   
            return 0;  
        case 1:  
            return num[0];  
        case 2:  
            return num[0] + num[1];  
        default:  
            break;  
        }  
        sort(num.begin(), num.end());  
        int closet = 0;  
        int sum = 0;  
        int i = 0, j = 0, k = num.size()-1;  
        int diff = INT_MAX;  
  
        for (i = 0; i < k-1; i++)  
        {  
            for (j = i+1; j < k;)  
            {  
                sum = num[i] + num[j] + num[k];  
                if (sum == target)  
                {  
                    return sum;  
                }  
                if (abs(sum-target) < diff)  
                {  
                    closet = sum;  
                    diff = abs(sum - target);  
                }  
                if (sum < target)  
                {  
                    j++;  
                }  
                else  
                {  
                    k--;  
                }  
            }  
        }  
        return closet;  
    }  
};