[LeetCode] SUM4

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

Note:

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

    For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

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

和sum3 差不多，枚举a ,b  再 解 c+d 的定值的问题。 

 写的时候漏了一些小地方， continue 之前要 改变p,q .

class Solution {
private:
	vector<vector<int> > ret;
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
		ret.clear();
		sort(num.begin(),num.end());
		for(int i  = 0; i < num.size() ; i++)
		{
			if( i > 0 && num[i] == num[i-1])
				continue;
			for( int j = i+1; j < num.size() ; j++)
			{
				if( j > i+1 && num[j]==num[j-1])
					continue;
				int p = j + 1;
				int q = num.size() -1;
				while( p < q)
				{
					if( p > j + 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[j] + num[p] + num[q];
					if(sum==target)
					{
						vector<int> tmp;
						tmp.push_back(num[i]);
						tmp.push_back(num[j]);
						tmp.push_back(num[p]);
						tmp.push_back(num[q]);
						ret.push_back(tmp);
						p++;
					}else if(sum > target)
					{
							q--;
					}else
					{
							p++;
					}
					
				}
			
			}
		}
		
       return ret; 
    }
};