[LeetCode] 18. 4Sum

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]
]

Thought process:
Add one loop to "15. 3Sum".

Solution:
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        List<List<Integer>> quadruplets = new ArrayList<>();
        
        for (int i = 0; i < nums.length - 3; i++) {
            while (i > 0 && i < nums.length - 3 && nums[i] == nums[i - 1]) {
                i++;
            }
            
            for (int j = i + 1; j < nums.length - 2; j++) {
                while (j > i + 1 && j < nums.length - 2 && nums[j] == nums[j - 1]) {
                    j++;
                }
                if (j >= nums.length) {
                    break;
                }
                
                int start = j + 1;
                int end = nums.length - 1;
                while (start < end) {
                    int fourSum = nums[i] + nums[j] + nums[start] + nums[end];
                    if (fourSum < target) {
                        start++;
                    } else if (fourSum > target) {
                        end--;
                    } else {
                        List<Integer> quadruplet = new ArrayList<>();
                        quadruplet.addAll(Arrays.asList(nums[i], nums[j], nums[start], nums[end]));
                        quadruplets.add(quadruplet);
                        
                        start++;
                        while (start < nums.length && nums[start] == nums[start - 1]) {
                            start++;
                        }
                        end--;
                        while (end >= 0 && nums[end] == nums[end + 1]) {
                            end--;
                        }
                    }
                }
            }
        }
        
        return quadruplets;
    }
}

Time complexity:
Sorting takes O(nlogn). Loop takes O(n^3). The overall time complexity is O(n^3).
Location: San Jose, CA, USA

Comments

Post a Comment

Popular posts from this blog

[LeetCode] 714. Best Time to Buy and Sell Stock with Transaction Fee

Your are given an array of integers  prices , for which the  i -th element is the price of a given stock on day  i ; and a non-negative integer  fee  representing a transaction fee. You may complete as many transactions as you like, but you need to pay the transaction fee for each transaction. You may not buy more than 1 share of a stock at a time (ie. you must sell the stock share before you buy again.) Return the maximum profit you can make. Example 1: Input: prices = [1, 3, 2, 8, 4, 9], fee = 2 Output: 8 Explanation: The maximum profit can be achieved by: Buying at prices[0] = 1 Selling at prices[3] = 8 Buying at prices[4] = 4 Selling at prices[5] = 9 The total profit is ((8 - 1) - 2) + ((9 - 4) - 2) = 8. Note: 0 < prices.length <= 50000 . 0 < prices[i] < 50000 . 0 <= fee < 50000 . Thought process: Each day there are two options, buy or sell. Use two variables buy and sell to keep track of the max profit. Solutio...
Read more

[LeetCode] 269. Alien Dictionary

There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of  non-empty  words from the dictionary, where  words are sorted lexicographically by the rules of this new language . Derive the order of letters in this language. Example 1: Given the following words in dictionary, [ "wrt", "wrf", "er", "ett", "rftt" ] The correct order is:  "wertf" . Example 2: Given the following words in dictionary, [ "z", "x" ] The correct order is:  "zx" . Example 3: Given the following words in dictionary, [ "z", "x", "z" ] The order is invalid, so return  "" . Note: You may assume all letters are in lowercase. You may assume that if a is a prefix of b, then a must appear before b in the given dictionary. If the order is invalid, return an empty string...
Read more

[LeetCode] 631. Design Excel Sum Formula

Your task is to design the basic function of Excel and implement the function of sum formula. Specifically, you need to implement the following functions: Excel(int H, char W):  This is the constructor. The inputs represents the height and width of the Excel form.  H  is a positive integer, range from 1 to 26. It represents the height.  W  is a character range from 'A' to 'Z'. It represents that the width is the number of characters from 'A' to  W . The Excel form content is represented by a height * width 2D integer array  C , it should be initialized to zero. You should assume that the first row of  C  starts from 1, and the first column of  C  starts from 'A'. void Set(int row, char column, int val):  Change the value at  C(row, column)  to be val. int Get(int row, char column):  Return the value at  C(row, column) . int Sum(int row, char column, List of Strings : numbers):  This function...
Read more