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Check if All A's Appears Before All B's

Difficulty: Easy

Problem Description

Given a string s consisting of only the characters 'a' and 'b', return true if every 'a' appears before every 'b' in the string. Otherwise, return false.

Key Insights

  • The problem can be simplified by checking the order of characters in the string.
  • If we find an 'a' after we have already found a 'b', the condition is violated.
  • The string can contain zero or more 'a's followed by zero or more 'b's to return true.

Space and Time Complexity

Time Complexity: O(n) - We traverse the string once. Space Complexity: O(1) - We use a constant amount of space.

Solution

To solve this problem, we can iterate through the string while maintaining a flag to track whether we have encountered a 'b'. If we encounter an 'a' after having seen a 'b', we return false. If we can traverse the string without finding such a case, we return true. This approach only requires a single pass through the string, and we do not need any additional data structures.

Code Solutions

def checkString(s: str) -> bool:
    seen_b = False  # Flag to track if we've seen 'b'
    
    for char in s:
        if char == 'b':
            seen_b = True  # Mark that we have seen a 'b'
        elif char == 'a' and seen_b:
            return False  # Found 'a' after 'b', condition violated
    
    return True  # All 'a's are before all 'b's
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