[leetcode] 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).

该题目与3Sum类似，但并不是列出所有和为0的三元组，而是找出与target最相近的sum，同样是利用三个指针i，p和q，具体请参考3Sum。只不过在遍历时记录最小误差minerror，最后返回target+minerror即可。

代码如下：

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) 
    {
        sort(num.begin(), num.end());
        int minerror = num[0]+num[1]+num[2] - target;
        for( int i = 0 ; i < num.size() ; i++ )
        {
            if( i > 0 && num[i] == num[i-1] )
            {
                continue;
            }
            int p = i+1;
            int q = num.size() - 1;
            while( p < q )
            {
                if( p > i +1 && num[p] == num[p-1] )
                {
                    p++;
                    continue;
                }

                if( q < num.size() -1 &&  num[q] == num[q+1] )
                {
                    q--;
                    continue;
                }

                int sum = num[i] + num[p] + num[q]; 

                if( abs(sum - target) < abs(minerror) )
                {
                    minerror = sum - target;
                }

                if( sum - target  == 0)
                {
                    return target;
                }

                if( sum - target > 0 )
                {
                    q--;
                }

                if( sum - target < 0 )
                {
                    p++;
                }
            }
        }
            return minerror + target;
    }
};