15 3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• The solution set must not contain duplicate triplets.

For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)

class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
    	vector<vector<int> > result;
    	int len = num.size();
    	sort(num.begin(),num.end());
    	for(int i = 0;i < len - 2;++i)
    	{
    		if(i > 0 && num[i] == num[i - 1]) continue;
    		for(int j = i + 1,k = len - 1;j < k;)
    		{
    			if(j > i + 1 && num[j] == num[j - 1])
    			{
    				++j;
    				continue;
    			}
    			if(k < len - 1 && num[k] == num[k + 1])
    			{
    				--k;
    				continue;
    			}
    
    			int sum = num[i] + num[j] + num[k];
    			if(0 == sum)
    			{
    				vector<int> tmp;
    				tmp.push_back(num[i]);
    				tmp.push_back(num[j]);
    				tmp.push_back(num[k]);
    
    				++j;
    				--k;
    				result.push_back(tmp);
    			}
    			else if(sum < 0)
    			{
    				++j;
    			}
    			else
    			{
    				--k;
    			}
    		}
    	} 
    	
    	return result;
    }
};