4Sum

Question:

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)

Solution:

 1 class Solution {
 2 public:
 3     vector<vector<int>> fourSum(vector<int>& nums, int target) {
 4         vector< vector<int> > result;
 5     sort(nums.begin(),nums.end());
 6     if(nums.size()<4)
 7         return result;
 8     for(auto i=nums.begin();i!=nums.end()-3;i++)
 9     {
10         for(auto j=i+1;j!=nums.end()-2;j++)
11         {
12             auto p=j+1;
13             auto q=nums.end()-1;
14             while(p<q)
15             {
16                 if(*i+*j+*p+*q<target)
17                 {
18                     p++;
19                 }
20                 else if(*i+*j+*p+*q>target)
21                 {
22                     q--;
23                 }
24                 else
25                 {
26                     vector<int> temp;
27                     temp.push_back(*i);
28                     temp.push_back(*j);
29                     temp.push_back(*p);
30                     temp.push_back(*q);
31                     result.push_back(temp);
32                     q--;
33                 }
34             }
35         }
36     }
37     sort(result.begin(),result.end());
38     auto it=unique(result.begin(),result.end());
39     result.erase(it,result.end());
40     return result;
41         
42     }
43 };