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

LeetCode 2243 | Difficulty: Easy

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.

Example 1:

Input: s = "aaabbb"
Output: true
Explanation:
The 'a's are at indices 0, 1, and 2, while the 'b's are at indices 3, 4, and 5.
Hence, every 'a' appears before every 'b' and we return true.

Example 2:

Input: s = "abab"
Output: false
Explanation:
There is an 'a' at index 2 and a 'b' at index 1.
Hence, not every 'a' appears before every 'b' and we return false.

Example 3:

Input: s = "bbb"
Output: true
Explanation:
There are no 'a's, hence, every 'a' appears before every 'b' and we return true.

Constraints:

- `1 <= s.length <= 100`

- `s[i]` is either `'a'` or `'b'`.

Topics: String

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Approach

String Processing

Consider character frequency counts, two-pointer approaches, or building strings efficiently. For pattern matching, think about KMP or rolling hash. For palindromes, expand from center or use DP.

When to use

Anagram detection, palindrome checking, string transformation, pattern matching.

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Solutions

Solution 1: C# (Best: 68 ms)

Metric	Value
Runtime	68 ms
Memory	38.5 MB
Date	2022-02-13

Solution

public class Solution {
    public bool CheckString(string s) {
        int i =0, maxA = -1;
    for (; i < s.Length; i++)
    {
        if(s[i] == 'b') break;
    }
    while(++i<s.Length)
    {
        if(s[i]=='a') return false;
    }
    return true;
    }
}

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Complexity Analysis

Approach	Time	Space
Solution	$O(n)$	$O(1) to O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• LeetCode provides 2 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: You can check the opposite: check if there is a ‘b’ before an ‘a’. Then, negate and return that answer.

Hint 2: s should not have any occurrences of “ba” as a substring.