ValidPalindrome.java

package learn.freq05;

//检查一个string是否是回文 eg:racecar
//also we define empty string as valid palindrome.
//"a." 是回文 因为  considering only alphanumeric characters and ignoring cases. 
public class ValidPalindrome {

    public boolean isPalindrome(String s) {
        if ((s.trim()).equals("") || s == null) {
            return true;
        }
        // STRING 先要处理 去除非alphanueric的 和空格
        String temp = s.toLowerCase();
//		for (int i = 0; i < temp.length(); i++) {
//			if (!((temp.charAt(i) >= '0' && temp.charAt(i) <= '9') || (temp
//					.charAt(i) >= 'a' && temp.charAt(i) <= 'z'))) {
//				temp.replace(temp.charAt(i), ' ');
//				System.err.print(i);
//			}
//		}
//		System.err.print(temp);
//		String withoutSpaceString = temp.replace("$", "");
//
//		withoutSpaceString = withoutSpaceString.replace(" ", "");

        int start = 0;
        int end = temp.length() - 1;
        while (start <= end) {
            if (((temp.charAt(start) >= '0' && temp.charAt(start) <= '9') || (temp
                    .charAt(start) >= 'a' && temp.charAt(start) <= 'z'))
                    && ((temp.charAt(end) >= '0' && temp.charAt(end) <= '9') || (temp
                    .charAt(end) >= 'a' && temp.charAt(end) <= 'z'))) {
                if (temp.charAt(start) != temp.charAt(end)) {
                    return false;
                }
                start++;
                end--;
            } else {
                if (!((temp.charAt(start) >= '0' && temp.charAt(start) <= '9') || (temp
                        .charAt(start) >= 'a' && temp.charAt(start) <= 'z'))) {
                    start++;
                }
                if (!((temp.charAt(end) >= '0' && temp.charAt(end) <= '9') || (temp
                        .charAt(end) >= 'a' && temp.charAt(end) <= 'z'))) {
                    end--;
                }
            }
        }
        return true;

    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.print((new ValidPalindrome()).isPalindrome("avavba"));
        System.out.print((new ValidPalindrome()).isPalindrome2("ab"));
    }


    public boolean isPalindrome2(String s) {
        if (s == null) {
            return false;
        }
        if (s.length() == 0 || s.trim().equals("")) {
            return true;
        }
        s = s.toLowerCase();
        StringBuilder chars = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) >= 'a' && s.charAt(i) <= 'z' || s.charAt(i) >= '0' && s.charAt(i) <= '9') {
                chars.append(s.charAt(i));
            }
        }
        String string = chars.toString();
        System.err.println(string);
        if (string.length() == 0) {
            return true;
        }
        int start = 0;
        int end = string.length() - 1;
        System.err.println(start + "," + end);
        while (start <= end) {
            if (string.charAt(start) != string.charAt(end)) {
                return false;
            }
            start++;
            end--;
        }
        return true;
    }

}