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) 
    {
        int n=num.size();
        int res;
        int hold = -1;
        if(n<3)
            return  res;
        heap_sort(num);
        for(int i=0; i<n; i++)
        {
            int low=i+1,high=n-1;
            while(low < high)
            {
                int temp = num[low] + num[high] + num[i];
                if(target < temp)
                    high--;
                else if(target > temp)
                    low++;
                else
                    return target;
                if(hold == -1 || abs(target - temp) < hold)
                {
                    hold = abs(target - temp);
                    res = temp;
                }
            }
        }
        return res;
    }
    void swap(vector<int>& num, int i, int j)
    {
        int temp = num[i];
        num[i] = num[j];
        num[j] = temp;
    }
    void sink(vector<int>& num, int s, int end)
    {
        while(s < end)
        {
            int k = 2*s+1;
            if(k>end)
                return;
            if(k<end && num[k+1] > num[k])
                k++;
            if(num[s] > num[k])
                return;
            swap(num,s,k);
            s = k;
        }
    }
    void heap_sort(vector<int>& num)
    {
        int n = num.size()-1;
        int i;
        for(i=(n-1)/2; i>=0; i--)
            sink(num,i,n);
        i = n;
        while(i>0)
        {
            swap(num,0,i);
            i--;
            sink(num,0,i);
        }
    }
};

LeetCode--3Sum Closest