LeetCode 3908: Valid Digit Number

Meta Data

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Difficulty: easy First Attempt: 2026-04-25

• Submissions: Accepted (first try)

Approach

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Both conditions are string operations:

• Contains x at least once: str(x) in s.
• Does not start with x: not s.startswith(str(x)).

My Solution

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class Solution:
    def validDigit(self, n: int, x: int) -> bool:
        s = str(n)
        return str(x) in s and not s.startswith(str(x))

Complexity

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Time O(d), space O(d).

Mistakes to Avoid

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• x is an integer; convert it to a string before comparing with strings.

Interview Explanation

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I convert n to a string and check that it contains x and that its first character is not x. Time O(d).