Is Check If Word Is Valid After Substitutions easy or hard?

The problem is rated Medium on LeetCode with an acceptance rate around 60%. The difficulty comes from recognizing that the string represents nested or sequential "abc" constructions and that a stack can validate the pattern in linear time.

Check If Word Is Valid After Substitutions Python/Java solution

In Python or Java, the typical implementation uses a stack (or list/ArrayDeque). Iterate through the string, push characters, and when 'c' appears verify the top of the stack contains 'b' and 'a'. Remove them to simulate eliminating "abc". The algorithm runs in O(n) time and O(n) space.

How to solve Check If Word Is Valid After Substitutions in O(n)?

Process the string with a stack. Push characters as you iterate, and when you see 'c', check if the top two elements form 'a' followed by 'b'. If they do, pop them and discard the 'c', effectively removing "abc". Continue until the string ends. The string is valid only if the stack is empty at the end.

What is the best approach for Check If Word Is Valid After Substitutions?

The stack-based validation approach is the most efficient and commonly expected solution. Iterate through the string and push characters onto a stack. Whenever you encounter 'c', check whether the previous two stack elements are 'a' and 'b'. If so, remove them to simulate eliminating the substring "abc". This runs in O(n) time and O(n) space.

Is Check If Word Is Valid After Substitutions asked at Google/Amazon/Meta?

This problem reflects common string validation patterns frequently used in interviews at companies like Amazon, Google, and Meta. It tests stack usage, pattern recognition, and linear-time string processing—skills often evaluated in mid-level algorithm rounds.

What data structure is used in Check If Word Is Valid After Substitutions?

A stack is the primary data structure used in the optimal solution. The stack tracks partially formed sequences such as 'a' or 'ab'. When a 'c' appears, the algorithm checks the stack to confirm whether a valid "abc" sequence has been completed.

What is the time complexity of Check If Word Is Valid After Substitutions?

The optimal stack-based solution runs in O(n) time because each character is pushed and popped at most once. Space complexity is O(n) due to the stack storing intermediate characters. A simpler string replacement approach may take O(n^2) time because it repeatedly scans and rebuilds the string.

Description Solution Video 20:24

1003. Check If Word Is Valid After Substitutions

Medium61.1% acceptance1 company

Given a string s, determine if it is valid.

A string s is valid if, starting with an empty string t = "", you can transform t into s after performing the following operation any number of times:

Insert string "abc" into any position in t. More formally, t becomes t_left + "abc" + t_right, where t == t_left + t_right. Note that t_left and t_right may be empty.

Return true if s is a valid string, otherwise, return false.

Example 1:

Input: s = "aabcbc"
Output: true
Explanation:
"" -> "abc" -> "aabcbc"
Thus, "aabcbc" is valid.

Example 2:

Input: s = "abcabcababcc"
Output: true
Explanation:
"" -> "abc" -> "abcabc" -> "abcabcabc" -> "abcabcababcc"
Thus, "abcabcababcc" is valid.

Example 3:

Input: s = "abccba"
Output: false
Explanation: It is impossible to get "abccba" using the operation.

Constraints:

1 <= s.length <= 2 * 10⁴
s consists of letters 'a', 'b', and 'c'

Asked at 1 company

Nutanix

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Stack String

20:24

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1003. Check If Word Is Valid After Substitutions

Asked at 1 company

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