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)

 

Code:

class Solution {public:vector<vector<int> > ret;vector<vector<int> > fourSum(vector<int> &num, int target){ret.clear();sort(num.begin(),num.end());for(int i=0;i<num.size();i++){if(i!=0 && num[i]==num[i-1])continue;for(int j=i+1;j<num.size();j++){if(j!=i+1 && num[j]==num[j-1])continue;int k1=j+1;int k2=num.size()-1;while(k1<k2){if(k1>j+1 && num[k1]==num[k1-1]){k1++;continue;}if(k2<num.size()-1 && num[k2]==num[k2]+1){k2--;continue;}int sum=num[i]+num[j]+num[k1]+num[k2];if(sum==target){vector<int> vet;vet.push_back(num[i]);vet.push_back(num[j]);vet.push_back(num[k1]);vet.push_back(num[k2]);ret.push_back(vet);k1++;k2--;}else if(sum<target){k1++;}else{k2--;}}}}return ret;}};