LeetCode-in-Java
125. Valid Palindrome
Easy
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 * 105sconsists only of printable ASCII characters.
Solution
public class Solution {
public boolean isPalindrome(String s) {
int i = 0;
int j = s.length() - 1;
boolean res = true;
while (res) {
// Iterates through string to find first char which is alphanumeric.
// Done to ignore non-alphanumeric characters.
// Starts from 0 to j-1.
while (i < j && isNotAlphaNumeric(s.charAt(i))) {
i++;
}
// Similarly from j-1 to 0.
while (i < j && isNotAlphaNumeric(s.charAt(j))) {
j--;
}
// Checks if i is greater than or equal to j.
// The main loop only needs to loop n / 2 times hence this condition (where n is string
// length).
if (i >= j) {
break;
}
// Assigning found indices to variables.
// The upperToLower function is used to convert characters, if upper case, to lower
// case.
// If already lower case, it'll return as it is.
char left = upperToLower(s.charAt(i));
char right = upperToLower(s.charAt(j));
// If both variables are not same, result becomes false, and breaks out of the loop at
// next iteration.
if (left != right) {
res = false;
}
i++;
j--;
}
return res;
}
private boolean isNotAlphaNumeric(char c) {
return (c < 'a' || c > 'z') && (c < 'A' || c > 'Z') && (c < '0' || c > '9');
}
private boolean isUpper(char c) {
return c >= 'A' && c <= 'Z';
}
private char upperToLower(char c) {
if (isUpper(c)) {
c = (char) (c + 32);
}
return c;
}
}