Marc Lester Tan

Just another easy problem.

class Solution {
    fun findTheDifference(s: String, t: String): Char {
        var chars = IntArray(26)
        t.forEach {
            chars[it - 'a']++
        }
        
        s.forEach {
            chars[it - 'a']--
        }
        
        chars.forEachIndexed { index, value ->
            if(value > 0) return (index+97).toChar()
        }
        
        throw IllegalStateException("Should not happen")
    }
}

Runtime: 156 ms, faster than 86.54% of Kotlin online submissions for Find the Difference.
Memory Usage: 32.7 MB, less than 100.00% of Kotlin online submissions for Find the Difference.

The idea is to have a character histogram of t and then for each character of s, reduce the matching bins in the histogram. Any bin whose count is not zero represents the added character.