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 * 105
• s consists 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
    }
}

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