[Leetcode 18]四数之和 4 Sum

Posted 2021-01-20 leetcode刷题中

【题目】

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.

【思路】

和leetcode15三数之和3 Sum类似https://www.cnblogs.com/inku/p/9955638.html

多一次循环，加粗部分是新增的

重点是去重。

【代码】

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> data=new ArrayList<>();
        Arrays.sort(nums);
        for(int i=0;i<nums.length-2;i++){
            for(int j=nums.length-1;j>0;j--){
                int left=i+1;
                int right=j-1;
                if(i>0&&nums[i]==nums[i-1]){
                    continue;}
                if(j<nums.length-1&&nums[j]==nums[j+1]){
                    continue;}
                while(left<right){
                    int sum=nums[left]+nums[right]+nums[i]+nums[j];
                    if(sum==target){
                        data.add(Arrays.asList(nums[left], nums[right]，nums[i],nums[j]));
                        left++;
                        right--;
                        while(left<right&&nums[left]==nums[left-1])
                            left++;
                        while(left<right&&nums[right]==nums[right+1])
                            right--;
                    }
                    else if(left<right&&sum>target)
                        right--;
                    else
                        left++;
                }
            }
        }
        return data;
    }
}