Leetcode - Episode 2 - Three More Easys

January 02, 2019 in Algorithms

After warming up (literally, I cycled home in frosty weather) I tackled some more Leetcode problems today.

961. N-Repeated Element in Size 2N Array

Problem: Given an list A of size 2N, there are N+1 unique elements — which one is repeated N times?

For this solution I used the high performance Counter that I discovered in my previous blog post.

from collections import Counter

class Solution:
    def repeatedNTimes(self, A):
        :type A: List[int]
        :rtype: int
        counter = set()
        half = len(A) / 2

        for i in A:
            counter[i] += 1
            if counter[i] == half:

                return i

This solution has a runtime complexity of O(n) and a space complexity of O(n). I’m not sure whether this can be improved upon. All elements of the list need to be checked because it is unsorted — we can’t know where our N element will be.

Leetcode’s tests for this problem only have unique elements and so a Set is technically faster but I prefer my solution as it’s more ‘correct’ per the problem statement.

965. Univalued Binary Tree

Problem: Are all the values of this binary tree the same?

It’s a tree-traversal problem so it’s either going to be Depth-First Search or Breadth-First Search. I chose DFS.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def isUnivalTree(self, root):
        :type root: TreeNode
        :rtype: bool
        value = root.val
        def recursive_search(node):
            if node == None:
                return True
            elif node.val != value:
                return False
                return recursive_search(node.left) and recursive_search(node.right)
            return True
        return recursive_search(root)

Whatever the solution to this problem, the runtime complexity will be O(n) as every node must be touched to satisfy the problem condition. We used DFS so the stack will only hold one singular winding tree-root in memory at once. Hence the space complexity is O(d) where d is the depth of the tree.

804. Unique Morse Code Words

Problem: Given a list of lowercase words, how many unique morse code sequences are there?

They’re looking for unique occurrences so a standard optimization is likely to be a Set.

class Solution:
    def uniqueMorseRepresentations(self, words):
        :type words: List[str]
        :rtype: int
        morse = [".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."]

        unique_words = set()
        for word in words:
            as_morse = []
            for c in word:
                as_morse.append(morse[ord(c) - 97])

        return len(unique_words)

My solution has a runtime complexity of O(n) with a space complexity of O(n). Similar to Leetcode problem 709, it may be faster to use a Dictionary that maps letters to morse as opposed to performing a calculation for each letter but this may depend on language implementation.

After looking through other peoples’ Python solutions, I noted that for the future I need to focus on using more Python language features. For instance, I saw lines like:

code = ''.join([morseTable[ord(letter) - 97] for letter in list(word)]).

Lines like this are more terse while remaining understandable.

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