LeetCode??4Sum

发布于:2021-09-26 08:44:12

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]
]
解法

与3sum类似,只是多加一层循环


public List> fourSum(int[] nums, int target) {
List> res=new ArrayList>();
List list;
if(nums.length<4)
return res;
Arrays.sort(nums);
for(int i=0;i {
if(i>0&&nums[i]==nums[i-1])
continue;
for(int j=i+1;j {
if(j>i+1&&nums[j]==nums[j-1])
continue;
int start=j+1,end=nums.length-1;
while(start {
int sum=nums[i]+nums[j]+nums[start]+nums[end];
if(sum==target)
{
list=new ArrayList();
list.add(nums[i]);
list.add(nums[j]);
list.add(nums[start]);
list.add(nums[end]);
res.add(list);
while(start start++;
while(start end--;
start++;
end--;
}
else if(target end--;
else
start++;
}
}
}
return res;
}

Runtime: 33 ms, faster than 78.19% of Java online submissions for 4Sum.
Memory Usage: 40.6 MB, less than 55.37% of Java online submissions for 4Sum.
leetcode上给的提示中写了HashTable,但是我百度了好像没有找到用HashTable做的方法,我自己尝试了一下,但是没办法排除重复性,希望有知道怎么做或者知道哪里有这种做法的人可以一起来交流一下

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