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)

Solution:

class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int> > res;
int len = nums.size();
if(len < 4) return res;
sort(nums.begin(), nums.end());
for(int i = 0; i < len; ++i)
{
if(i != 0 && nums[i] == nums[i-1]) continue;
for(int j = i + 1; j < len; ++j)
{
if(j != i + 1 && nums[j] == nums[j-1]) continue;
int left = j + 1, right = len - 1;
int mid = target - nums[i] - nums[j];
vector<int> v;
v.push_back(nums[i]);
v.push_back(nums[j]);
while(left < right)
{
int l = nums[left], r = nums[right];
if(l + r == mid)
{
v.push_back(l);
v.push_back(r);
res.push_back(v);
v.pop_back();
v.pop_back();
while(nums[++left] == l) ;
while(nums[--right] == r) ;
}
else if(l + r < mid)
{
while(nums[++left] == l) ;
}
else
{
while(nums[--right] == r) ;
}
}
}
}

return res;
}
};