Leetcode: 125. Valid Palindrome(JAVA)

【问题描述】

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.

【思路】

使用两个指针，i指向头，j指向尾，依次向中间移动，进行比较，时间复杂度为O(n)

【code】

public class Solution {
    public boolean isPalindrome(String s) {
       		int i = 0, j = s.length() - 1;
		s = s.toLowerCase();
		while (i < j) {
			char c = s.charAt(i);
			char cc = s.charAt(j);
			if (!isValid(c)) {
				i++;
			}else if (!isValid(cc)) {
				j--;
			}else {
				if (c == cc) {
					i++;
					j--;
				}else {
					return false;
				}
			}
		}
		return true;
	}

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