1003. Check If Word Is Valid After Substitutions
Description
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 int
. More formally,t
becomestleft + "abc" + tright
, wheret == tleft + tright
. Note thattleft
andtright
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'
Solutions
Solution 1: Stack
We observe the operations in the problem and find that each time a string $\textit{"abc"}$ is inserted at any position in the string. Therefore, after each insertion operation, the length of the string increases by $3$. If the string $s$ is valid, its length must be a multiple of $3$. Thus, we first check the length of the string $s$. If it is not a multiple of $3$, then $s$ must be invalid, and we can directly return $\textit{false}$.
Next, we traverse each character $c$ in the string $s$. We first push the character $c$ onto the stack $t$. If the length of the stack $t$ is greater than or equal to $3$, and the top three elements of the stack form the string $\textit{"abc"}$, then we pop the top three elements from the stack. We then continue to traverse the next character in the string $s$.
After the traversal, if the stack $t$ is empty, it means the string $s$ is valid, and we return $\textit{true}$; otherwise, we return $\textit{false}$.
The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the length of the string $s$.
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