leetcode [#125]

目录

• 1. 题目
• 2. 解决方案
• 3. 注意事项

题目

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

Example
“A man, a plan, a canal: Panama” is a palindrome.
“race a car” is not a palindrome.

Note
Have you consider that the string might be empty? This is a good question to ask during an interview.

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

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解决方案

public class Solution {
    public boolean isPalindrome(String s) {
        int len = s.length();
        if(s == null || s.length() == 0) return true;
        StringBuilder builder1 = new StringBuilder();
        StringBuilder builder2 = new StringBuilder();
        s = s.toLowerCase();
        boolean result = true;
        for(int i = 0; i < len; i++){
            if(isDigitOrLetter(s.charAt(i))){
                builder1.append(s.charAt(i));
            }
        }
        for(int j = len-1 ; j >= 0; j--){
            if(isDigitOrLetter(s.charAt(j))){
                builder2.append(s.charAt(j));
            }
        }
        for(int k = 0; k < builder1.length(); k++){
            if(builder1.charAt(k) != builder2.charAt(k)){
                result = false;
                break;
            }
        }
        return result;
    }
    private static boolean isDigitOrLetter(char s){
        if((s >= 'a' && s <= 'z') || (s >= 'A' && s <= 'Z') ||(s >= '0' && s <= '9')){
            return true;
        } else {
            return false;
        }
    }
}

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注意事项

1. 先处理特殊情况，字符串为空或者长度为零。
2. 定义一个函数isDigitOrLetter()来判断某个字符是否是字母大小写+数字。
3. 使用两个StringBuilder，分别从首尾遍历字符串，存储为字母大小写+数字的字符。
4. 比较两个StringBuilder中内容是否一样即可。