After warming up (literally, I cycled home in frosty weather) I tackled some more Leetcode problems today.
Problem: Given an list
A of size
2N, there are
N+1 unique elements — which one is repeated
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.
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 else: 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
d is the depth of the tree.
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]) unique_words.add(''.join(as_morse)) 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.
Comments or questions? I enjoy talking with readers over email.