[LeetCode] 125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

有效的回文。题意很简单，验证给的input是否是一个有效的回文。思路也很简单，two pointer逼近。注意

• 只比较input里面的字母和数字
• 需要ignore大小写

时间O(n)

空间O(1)

JavaScript实现

/**
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function (s) {
    if (s === null || s.length === 0) return true;
    const alphanum = s.toLowerCase().replace(/[W]/g, "");
    let left = 0;
    let right = alphanum.length - 1;
    while (left < right) {
        if (alphanum[left] !== alphanum[right]) {
            return false;
        }
        left++;
        right--;
    }
    return true;
};

Java实现

class Solution {
    public boolean isPalindrome(String s) {
        // corner case
        if (s == null || s.length() == 0) {
            return true;
        }

        // normal case
        int left = 0;
        int right = s.length() - 1;
        while (left < right) {
            while (left < right && !Character.isLetterOrDigit(s.charAt(left))) {
                left++;
            }
            while (left < right && !Character.isLetterOrDigit(s.charAt(right))) {
                right--;
            }
            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}

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LeetCode 题目总结