[LeetCode] 128. Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.
Your algorithm should run in O(n) complexity.

Thought process:
Iterate through the array, use a hashset to store all the numbers. Iterate through the array again, when I see a number, I check if number-- is in the set until it's not. If it's in the set, remove it from the set. Check number++ in the same fashion. This gives me O(n) time complexity because every number is iterated at most once.

Solution 1 (Set):
 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
public class Solution {
    public int longestConsecutive(int[] nums) {
        Set<Integer> set = new HashSet<>();
        for (int num : nums) {
            set.add(num);
        }
        
        int longest = 0;
        for (int num : nums) {
            int currentLongest = 0;
            int temp = num;
            while (set.contains(num)) {
                currentLongest++;
                set.remove(num);
                num--;
            }
            num = temp + 1;
            while (set.contains(num)) {
                currentLongest++;
                set.remove(num);
                num++;
            }
            longest = Math.max(longest, currentLongest);
        }
        
        return longest;
    }
}

The question can also be solved using union find. In order to find a number's neighbors quickly, we need to use a hash map as the union find data structure.

Solution 2 (Union find):
 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
48
49
50
51
class Solution {
    private int longest = 1;
    private Map<Integer, Integer> parent = new HashMap<>();
    private Map<Integer, Integer> size = new HashMap<>();
    
    public int longestConsecutive(int[] nums) {
        if (nums.length == 0) {
            return 0;
        }
        
        for (int num : nums) {
            if (parent.containsKey(num)) {
                continue;
            }
            
            parent.put(num, num);
            size.put(num, 1);
            if (parent.containsKey(num - 1)) {
                union(num, num - 1);
            }
            if (parent.containsKey(num + 1)) {
                union(num, num + 1);
            }
        }
        return longest;
    }
    
    private void union(int a, int b) {
        int p1 = find(a);
        int p2 = find(b);
        int size1 = size.get(p1);
        int size2 = size.get(p2);
        
        if (size1 < size2) {
            parent.put(p1, p2);
            size.put(p2, size1 + size2);
        } else {
            parent.put(p2, p1);
            size.put(p1, size1 + size2);
        }
        longest = Math.max(longest, size1 + size2);
    }
    
    private int find(int i) {
        while (parent.get(i) != i) {
            parent.put(i, find(parent.get(i)));
            i = parent.get(i);
        }
        return i;
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, 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