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)

此题和3sum思路基本相同，不同的是，需要固定两个数，而不是一个数，而且要求4个数的和是target而不是0。

class Solution {
private:
vector<vector<int>> result;
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
if(nums.size()<4)
return result;
sort(nums.begin(),nums.end());
vector<int> temp(4);
for(int i=0;i<nums.size()-3;i++){
if(i!=0&&nums[i]==nums[i-1])
continue;
for(int j=i+1;j<nums.size()-2;j++){
if(j!=i+1&&nums[j]==nums[j-1])
continue;
int left=j+1;
int right=nums.size()-1;
while(left<right){
int sum=nums[i]+nums[j]+nums[left]+nums[right];
if(sum==target){
temp[0]=nums[i];
temp[1]=nums[j];
temp[2]=nums[left];
temp[3]=nums[right];
result.push_back(temp);
left++;
right--;
while(left<right&&nums[left]==nums[left-1])
left++;
while(left<right&&nums[right]==nums[right+1])
right--;
}
else if(sum>target)
right--;
else
left++;
}
}

}
return result;
}
};