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).

class  Solution {
public:
     int threeSumClosest(vector< int> &num,  int target) 
    {
         for( int i= 0;i<num.size();i++)
             for( int j=i+ 1;j<num.size();j++)
                 if(num[i]>num[j])
                {
                     int tmp=num[i];
                    num[i]=num[j];
                    num[j]=tmp;
                }
         int size=num.size();
         if(size< 3)  return  0;
        
         int close=num[ 0]+num[ 1]+num[ 2];
         for( int index1= 0;index1<size- 2;index1++)
        {
             int index2=index1+ 1;
             int index3=size- 1;
             while(index2<index3)
            {
                 int sum=num[index1]+num[index2]+num[index3];
                 if(abs(close-target)>abs(sum-target))
                    close=sum;
                 if(sum<target) index2++;
                 else index3--;
            }
        }
         return close;
    }
     int abs( int a)
    {
         return a>= 0?a:-a;
    }
};