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125. Valid Palindrome

Posted on Edited on In Disqus:
Symbols count in article: 627 Reading time ≈ 1 mins.

O(n) time O(1) space

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class Solution {
public:
    bool isPalindrome(string s) {
        for (int l = 0, r = s.length() - 1; l < r; ++l, --r) {
            while (l < r && !isalnum(s[l])) ++l;
            while (l < r && !isalnum(s[r])) --r;
            if (tolower(s[l]) != tolower(s[r])) return false;
        }
        return true;
    }
};
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class Solution {
public:
    bool isPalindrome(string s) {
        int n = s.length();
        int l = 0, r = n - 1;
        while (l < r) {
            while (l < r && !isalnum(s[l])) {
                ++l;
            }
            while (l < r && !isalnum(s[r])) {
                --r;
            }
            if (tolower(s[l]) != tolower(s[r])) return false; // uniform case!!
            ++l;
            --r;
        }
        return true;
    }
};