3Sum

Given an array S of n integers, are there elements a,
b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

Elements in a triplet (a,b,c) must be in non-descending order. (ie,
a ≤ b ≤ c)
The solution set must not contain duplicate triplets.

For example, given array S = {-1 0 1 2 -1 -4},

A solution set is:
(-1, 0, 1)
(-1, -1, 2)

Solution:

class Solution {
public:
vector<vector<int>> threeSum(vector<int>& nums) {
vector<vector<int> > res;
int len = nums.size();
if(len < 3) return res;
sort(nums.begin(), nums.end());
for(int i = 0; i < len; ++i)
{
if(nums[i] > 0) break;
if(i > 0 && nums[i] == nums[i-1]) continue;
int sum = 0 - nums[i];
int left = i + 1, right = len - 1;
vector<int> v;
v.push_back(nums[i]);
while(left < right)
{
int l = nums[left];
int r = nums[right];
if(l + r == sum)
{
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(nums[left] + nums[right] > sum)
{
while(nums[--right] == r) ;
}
else
{
while(nums[++left] == l) ;
}
}
}

return res;
}
};