【LeetCode】18. 4Sum

18. 4Sum

Total Accepted: 68944 Total Submissions: 292188 Difficulty: Medium

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)

【解法及注释】

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> result;
		if(nums.size()<4)
			return result;
		
		int start,end;
		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;
				
				start=j+1;
				end=nums.size()-1;
				while(start<end)
				{
					int sum=nums[i]+nums[j]+nums[start]+nums[end];
				    if(sum==target)
				     {
				         
					    vector<int> fourNum;
					    fourNum.push_back(nums[i]);
					    fourNum.push_back(nums[j]);
					    fourNum.push_back(nums[start]);
					    fourNum.push_back(nums[end]);
					    result.push_back(fourNum);
					    fourNum.clear();
					       start++;
					       end--;
					       while(start<end&&nums[start]==nums[start-1])start++;
					       while(start<end&&nums[end]==nums[end+1])end--;
				     }
				    else if(sum>target)
				        end--;
				    else
					    start++;
				}
				
			}
		}
		return result;
    }
};