LeetCode in Kotlin
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
@Suppress("NAME_SHADOWING")
class Solution {
fun isPalindrome(s: String): Boolean {
var i = 0
var j = s.length - 1
var 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[i])) {
i++
}
// Similarly from j-1 to 0.
while (i < j && isNotAlphaNumeric(s[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.
val left = upperToLower(s[i])
val right = upperToLower(s[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 fun isNotAlphaNumeric(c: Char): Boolean {
return (c < 'a' || c > 'z') && (c < 'A' || c > 'Z') && (c < '0' || c > '9')
}
private fun isUpper(c: Char): Boolean {
return c >= 'A' && c <= 'Z'
}
private fun upperToLower(c: Char): Char {
var c = c
if (isUpper(c)) {
c += 32
}
return c
}
}