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
    
     
       > ret;public:    vector
      
       
         > fourSum(vector
        
          &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
         
           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; }};