[LeetCode]4Sum

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)

思考：在3Sum的基础上，再加一个循环。

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
		sort(num.begin(),num.end());
		int i,j,k,m,sum;
		vector<vector<int> > ret;
		for(i=0;i!=num.size();i++)
		{
			if(i&&num[i]==num[i-1]) continue;
			for(j=i+1;j!=num.size();j++)
			{
				if(j>i+1&&num[j]==num[j-1]) continue; 
				k=j+1;
				m=num.size()-1;
				while(k<m)
				{
					if(k>j+1&&num[k]==num[k-1])
					{
						k++;
						continue;
					}
					if(m<num.size()-1&&num[m]==num[m+1])
					{
						m--;
						continue;
					}
					sum=num[i]+num[j]+num[k]+num[m];
					if(sum==target)
					{
						vector<int> ans;
						ans.push_back(num[i]);
						ans.push_back(num[j]);
						ans.push_back(num[k]);
						ans.push_back(num[m]);
						ret.push_back(ans);
						k++;
						m--;
					}
					else if(sum<target) k++;
					else m--;				
				}
			}
		}
		return ret;
    }
};