[array] leetCode-18. 4Sum -Medium

18. 4Sum -Medium

descrition

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]
]

解析

与 3Sum 的思路一样。不过代码量一大要 bug free 还真得细心一些！！

code

#include <iostream>
#include <vector>
#include <algorithm>
#include <limits>

using namespace std;

class Solution{
public:
	int threeSumClosest(vector<int>& nums, int target){
		sort(nums.begin(), nums.end()); // ascending

		int min_gab = numeric_limits<int>::max();
		int ans = target;

		for(int i=0; i<nums.size(); i++){
			int target_local = target - nums[i];
			int ileft = i + 1;
			int iright = nums.size() - 1;
			while(ileft < iright){ // two pointer searching
				int sum = nums[ileft] + nums[iright];
				if(sum == target_local) // right answer
					return target;
				if(sum < target_local) // move ileft to increase sum
					ileft++;
				else // sum > target_local
					iright--;

				int gab = abs(sum - target_local);
				if(gab < min_gab){
					ans = sum + nums[i];
					min_gab = gab;
				}
			}
		}

		return ans;

	}
};

int main()
{
	return 0;
}