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)

Difficulty: Medium

思路不是很难，可是Debug真是一件无比痛！苦！的事，足足花了我两个小时，可能还是不够熟练吧。

vector<vector<int> > fourSum(vector<int>& nums, int target) {
vector<vector<int> > ans;
int len = nums.size();
if(len<4)
return ans;
sort(nums.begin(),nums.end());
for(int p1 = 0; p1<len-3;p1++)
{
for(int p4 = p1+1; p4<len-2; p4++)
{
int p2 = p4+1, p3 = len-1;
if(nums[p4]==nums[p4-1]&&p4-1>p1)
continue;
if(nums[p1]==nums[p1-1]&&p1>0)
continue;
while(p3>p2)
{
int target2 = target - nums[p1] - nums[p4];

if(nums[p2]==nums[p2-1]&&p2-1>p4)
{
p2++;
continue;
}

if(nums[p3]==nums[p3+1]&&p3+1<len-1)
{
p3--;
continue;
}

if(nums[p3]+nums[p2]>target2)
{
p3--;
continue;
}
else if(nums[p3]+nums[p2]<target2)
{
p2++;
continue;
}
else
{
vector<int> tmp;
int test;
tmp.push_back(nums[p1]);
tmp.push_back(nums[p4]);
tmp.push_back(nums[p2]);
tmp.push_back(nums[p3]);
ans.push_back(tmp);
p2++;
continue;
}
}

}
}
return ans;
}