A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.
Given a string s, return true if it is a palindrome, or false otherwise.
Example 1:
Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.
Example 2:
Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.
Example 3:
Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.
Constraints:
1 <= s.length <= 2 * 105
s consists only of printable ASCII characters.
Solution:
假定有指针i, j, 其中i是从前往后遍历, j是从后往前遍历.
当i在j左边时继续循环, 每一次将i右移到数字/字母上, j左移到数字/字母上,
比较二者对应的字符串内的字符是否相同, 不相同则原字符串不是回文串.
如果全部的比较都相同, 说明是回文串.
lass Solution {
public boolean isPalindrome(String s) {
if (s == null || s.length() == 0) {
return true;
}
s = s.toLowerCase();
int i = 0;
int j = s.length()-1;
while(i<j)
{
while(i < s.length() && !isValid(s.charAt(i)))
{i++;}
while(j >= 0 && !isValid(s.charAt(j)))
{j--;}
if(i< j && s.charAt(i) != s.charAt(j))
{
return false;
}
i++;
j--;
}
return true;
}
public boolean isValid(char c)
{
return Character.isLetter(c) || Character.isDigit(c);
}
}