4Sum

https://leetcode.com/problems/4sum/description/

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]

]

Thoughts

和3sum同理，只是要多加一层循环。

Code

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

        return res;
    }
}

Analysis

Errors:

1. while (l < r && nums[r] == nums[r + 1]) 没写在else里

2. if (j > 0 && nums[j] == nums[j - 1]) 导致[0,0,0,0]没有执行

3. 忘了sort

时间复杂度O(n^3)

做题耗时:20min