LeetCode 4Sum

题目

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)

 

循环求之前的3SUM问题，n^3

 

代码：

class Solution {public:    vector<vector<int> > fourSum(vector<int> &num, int target) {vector<vector<int>> ans;int size=num.size();if(size<4)return ans;sort(num.begin(),num.end());long long sum;vector<int> qua;int i=0,j=1,k=2,l=size-1;while(k<l){while(k<l)//3SUM{while(k<l)//2SUM{sum=num[i]+num[j]+num[k]+num[l];if(sum==target){qua.clear();qua.push_back(num[i]);qua.push_back(num[j]);qua.push_back(num[k]);qua.push_back(num[l]);ans.push_back(qua);k++;while(k<l&&num[k]==num[k-1])k++;l--;while(k<l&&num[l]==num[l+1])l--;}else if(sum<target)k++;elsel--;}j++;while(j<size&&num[j]==num[j-1])j++;k=j+1;l=size-1;}i++;while(i<size&&num[i]==num[i-1])i++;j=i+1;k=i+2;l=size-1;}return ans;    }};