Valid Palindrome II Python Solution with Tests

Tested Python solution for LeetCode 680 with 18 pytest cases. Generate a practice environment with lcpy.

LeetCode 680, Easy. Topics: Two Pointers, String, Greedy. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 18 parametrized pytest cases, and a playground notebook:

lcpy gen -n 680   # by problem number
lcpy gen -s valid_palindrome_ii   # by problem name

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Problem

Given a string s, return true if the s can be palindrome after deleting at most one character from it.

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Examples

Input: s = "aba"
Output: true

Input: s = "abca"
Output: true
Explanation: You could delete the character 'c'.

Input: s = "abc"
Output: false

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Constraints

• 1 <= s.length <= 10^5
• s consists of lowercase English letters.

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(n)
    # Space: O(1)
    def valid_palindrome(self, s: str) -> bool:
        def is_palindrome(left: int, right: int) -> bool:
            while left < right:
                if s[left] != s[right]:
                    return False
                left += 1
                right -= 1
            return True

        left, right = 0, len(s) - 1
        while left < right:
            if s[left] != s[right]:
                # Mismatch: try skipping either the left or the right character.
                return is_palindrome(left + 1, right) or is_palindrome(left, right - 1)
            left += 1
            right -= 1
        return True

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Complexity

Time	Space
O(n)	O(1)

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Tags

NeetCode 250, NeetCode All.