[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;
}
};