125. Valid Palindrome

难度: Easy

题目描述

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. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

 

Example 1:

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.

 

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

解题思路

代码实现

解决方案

java

class Solution {
    public boolean isPalindrome(String s) {

        int n = s.length();

        for (int i = 0, j = n - 1; i < j;) {

            char cI = Character.toLowerCase(s.charAt(i));
            char cJ = Character.toLowerCase(s.charAt(j));
            if (!isValid(cI)) {
                i++;
                continue;
            }
            if (!isValid(cJ)) {
                j--;
                continue;
            }
            if (cI != cJ) {
                return false;
            }
            i++;
            j--;

        }

        return true;

    }

    private boolean isValid(char cI) {
        if ((cI >= '0' && cI <= '9') || (cI >= 'a' && cI <= 'z')) {
            return true;
        }
        return false;
    }
}