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;
}
}