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: 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:

Fix the first two numbers, and squeeze the last two numbers from left / right. 

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