LeetCode: Valid Palindrome 解题报告

Valid Palindrome

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

For 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.

SOLUTION 1:

左右指针往中间判断。注意函数： toLowerCase

/*
    SOLUTION 1: Iterator.
    */
    public boolean isPalindrome1(String s) {
        if (s == null) {
            return false;
        }
        
        int len = s.length();
        
        boolean ret = true;
        
        int left = 0;
        int right = len - 1;
        
        String sNew = s.toLowerCase();
        
        while (left < right) {
            // bug 1: forget a )
            while (left < right && !isNumChar(sNew.charAt(left))) {
                left++;
            }
            
            while (left < right && !isNumChar(sNew.charAt(right))) {
                right--;
            }
            
            if (sNew.charAt(left) != sNew.charAt(right)) {
                return false;
            }
            
            left++;
            right--;
        }
        
        return true;
    }
    
    public boolean isNumChar(char c) {
        if (c <= '9' && c >= '0' || c <= 'z' && c >= 'a' || c <= 'Z' && c >= 'A') {
            return true;
        }
        
        return false;
    }

SOLUTION 2:

引自http://blog.csdn.net/fightforyourdream/article/details/12860445 的解答，会简单一点儿。不用判断边界。

左右指针往中间判断。新技能GET: isLetterOrDigit 

/*
    SOLUTION 2: Iterator2.
    */
    public boolean isPalindrome(String s) {
        if (s == null) {
            return false;
        }
        
        int len = s.length();
        
        boolean ret = true;
        
        int left = 0;
        int right = len - 1;
        
        String sNew = s.toLowerCase();
        
        while (left < right) {
            // bug 1: forget a )
            if (!Character.isLetterOrDigit(sNew.charAt(left))) {
                left++;
            // bug 2: Line 67: error: cannot find symbol: method isLetterOrDigital(char)    
            } else if (!Character.isLetterOrDigit(sNew.charAt(right))) {
                right--;
            } else if (sNew.charAt(left) != sNew.charAt(right)) {
                return false;
            } else {
                left++;
                right--;
            }
        }
        
        return true;
    }

 

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/string/IsPalindrome_2014_1229.java