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Number of Changing Keys

Difficulty: Easy

Problem Description

You are given a 0-indexed string s typed by a user. Changing a key is defined as using a key different from the last used key. Return the number of times the user had to change the key. Modifiers like shift or caps lock won't be counted in changing the key.

Key Insights

  • A change of key occurs only when the character changes between upper and lower case.
  • Consecutive characters of the same letter, regardless of case, do not count as a key change.
  • We can iterate through the string while comparing each character with the last used key.

Space and Time Complexity

Time Complexity: O(n) - where n is the length of the string, as we need to traverse the string once.
Space Complexity: O(1) - we are using a fixed amount of extra space regardless of input size.

Solution

To solve this problem, we can utilize a simple iteration approach. We will keep track of the last character that was typed (ignoring case). For each character in the string, we will check if it is different from the last character (again, ignoring case). If it is different, we increment our change key counter. This approach requires a single pass through the string, making it efficient in both time and space.

Code Solutions

def count_key_changes(s: str) -> int:
    # Initialize the counter for key changes
    key_changes = 0
    
    # Initialize last character as None
    last_char = None

    for char in s:
        # Check if the current character is different from the last used key (case-insensitive)
        if last_char is None or char.lower() != last_char.lower():
            key_changes += 1  # Increment the key change counter
            last_char = char  # Update the last character used

    return key_changes
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