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:

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector< vector<int> > result;
    sort(nums.begin(),nums.end());
    if(nums.size()<4)
        return result;
    for(auto i=nums.begin();i!=nums.end()-3;i++)
    {
        for(auto j=i+1;j!=nums.end()-2;j++)
        {
            auto p=j+1;
            auto q=nums.end()-1;
            while(p<q)
            {
                if(*i+*j+*p+*q<target)
                {
                    p++;
                }
                else if(*i+*j+*p+*q>target)
                {
                    q--;
                }
                else
                {
                    vector<int> temp;
                    temp.push_back(*i);
                    temp.push_back(*j);
                    temp.push_back(*p);
                    temp.push_back(*q);
                    result.push_back(temp);
                    q--;
                }
            }
        }
    }
    sort(result.begin(),result.end());
    auto it=unique(result.begin(),result.end());
    result.erase(it,result.end());
    return result;
        
    }
};