Leetcode - Episode 9 - Trudging Through (3x E)

The three-a-day streak nearly ended today.

I came close to completing a couple of Medium problems but I didn’t want to brute force the answers. I have a set of unwritten rules about what I’m allowed to look up in a book or google. All solutions need to be within the range of ‘optimal’ as well.

The goal is to get better at solving problems and algorithm analysis — not to get the green text that reads Accepted. I ended up searching for some Easys to complete so I could get to bed on time.

169. Majority Element

Problem: Given an array of size n find the majority element that appears more than n/2 times.

I wrote about the algoritm that solves this problem for the first post on this blog. The Boyer–Moore majority vote algorithm.

Let’s take a look at its linear runtime and constant space goodness.

class Solution:
    def majorityElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        counter = 1
        current = nums[0]
        for i in nums[1:]:
            if i != current:
                counter -= 1
                if counter < 0:
                    current = i
                    counter = 1
            else:
                counter += 1
        return current

Its all about tracking who is in front of the pack and swapping when a challenger comes along.

Runtime complexity: O(n).

Spacetime complexity: O(1).

520. Detect Capital

Problem: Judge whether a word uses correct capitals.

The three types of correctness: 'IRE', 'topcoder', and 'Bing'.

I wanted to test the runtime between using Python’s built-ins and a custom solution for Leetcode’s test cases.

So, the complex way:

class Solution:
    def detectCapitalUse(self, word):
        """
        :type word: str
        :rtype: bool
        """
        def is_cap(c):
            return ord(c) < 97
        
        if is_cap(word[0]) and len(word) > 1 and is_cap(word[1]):
            for c in word:
                if not is_cap(c):
                    return False
        elif is_cap(word[0]):
            for c in word[1:]:
                if is_cap(c):
                    return False
        else:
            for c in word[1:]:
                if is_cap(c):
                    return False

        return True

This had a runtime of 76ms.

Instead, we could have just written:

class Solution:
    def detectCapitalUse(self, word):
        """
        :type word: str
        :rtype: bool
        """
        return word.isupper() or word.islower() or word.istitle()

This much shorter version had a runtime of .. 76ms. The exact same result!

I’m fairly sure that as n tends to infinity, the more complex solution will surely win out.

Runtime complexity: O(n).

Space complexity: O(1).

620. Not Boring Movies

Problem: Given a SQL table cinema find movies with an odd numbered ID and a description that is not (literally) ‘boring’.

Not much to comment on this one. I read through the discussion board and some people found that using (not .. ) for their test expressions ran slower but I suppose that will be down to implementation details.

# Write your MySQL query statement below
SELECT * FROM cinema WHERE id % 2 != 0 AND description != 'boring' ORDER BY rating DESC;

Runtime complexity: O(n).

Spacetime complexity: O(n).