leetcode[18]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)

class Solution {
public:
vector<vector<int>> fourSum(vector<int> &num, int target) {
vector<vector<int>> res;
if(num.size()<4)return res;
sort(num.begin(),num.end());
int c,d;
for(int a=0;a<num.size();a++)
{
if(a>0&&num[a]==num[a-1])
continue;
for(int b=a+1;b<num.size();b++)
{
if(b>a+1&&num[b]==num[b-1])
continue;
c=b+1;
d=num.size()-1;
while(c<d)
{
if(c>b+1&&num[c]==num[c-1])
{
c++;
continue;
}
if(d<num.size()-1&&num[d]==num[d+1])
{
d--;
continue;
}
int sum=num[a]+num[b]+num[c]+num[d];
if(sum<target)c++;
else if(sum>target)d--;
else
{
vector<int> temp;
temp.push_back(num[a]);
temp.push_back(num[b]);
temp.push_back(num[c]);
temp.push_back(num[d]);
res.push_back(temp);
c++;
d--;
}
}
}
}
return res;
}
};