EasyLeetCode #125Two Pointers

Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Given a string s, return true if it is a palindrome, or false otherwise.

Constraints

1 <= s.length <= 2 * 10^5, s consists only of printable ASCII characters

Examples

Input: s = "A man, a plan, a canal: Panama"

Output: true

Solution

Approach

Use two pointers from both ends. Skip non-alphanumeric characters and compare lowercase versions.

def isPalindrome(s):
    left, right = 0, len(s) - 1
    while left < right:
        while left < right and not s[left].isalnum():
            left += 1
        while left < right and not s[right].isalnum():
            right -= 1
        if s[left].lower() != s[right].lower():
            return False
        left += 1
        right -= 1
    return True

Complexity

Time:O(n)

Space:O(1)

Hints

1.Use two pointers from start and end
2.Skip non-alphanumeric characters
3.Compare lowercase versions

Asked at

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