18.4sum.cpp

/*
 * [18] 4Sum
 *
 * https://leetcode.com/problems/4sum/description/
 *
 * algorithms
 * Medium (27.59%)
 * Total Accepted:    156.2K
 * Total Submissions: 565.9K
 * Testcase Example:  '[1,0,-1,0,-2,2]\n0'
 *
 * Given an array nums of n integers and an integer target, are there elements
 * a, b, c, and d in nums 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.
 * 
 * Example:
 * 
 * 
 * Given array nums = [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]
 * ]
 * 
 * 
 */
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
		vector<vector<int>> res;
		if (nums.size() < 4) return res;
		sort(nums.begin(),nums.end());
		
		for (int i = 0; i < nums.size() - 3; ++i){
			if (i > 0 && nums[i] == nums[i - 1]) continue;
			for (int j = i + 1; j < nums.size() -2; ++j){
				if (j > i + 1 && nums[j] == nums[j -1]) continue;
				int left = j + 1;
				int right = nums.size() - 1;
				int goal = target - nums[i] - nums[j];
				while(left < right){
					int temp = nums[left] + nums[right];
					if ( temp == goal){
						res.push_back({nums[i],nums[j],nums[left],nums[right]});
						while(left < right && nums[left] == nums[left + 1]) ++left;
						while(left < right && nums[right] == nums[right - 1]) --right;
						++left;
						--right;
					}else if (temp < goal)
					  ++left;
					else
					  --right;
				}
			}
		}
		return res;
    }
};