diff --git a/kotlin/0091-decode-ways.kt b/kotlin/0091-decode-ways.kt
index cad2fd1e5..96d315635 100644
--- a/kotlin/0091-decode-ways.kt
+++ b/kotlin/0091-decode-ways.kt
@@ -1,22 +1,22 @@
 class Solution {
-    fun numDecodings(s: String): Int {
-        val n = s.length
-        val dp = IntArray(n+1)
-        dp[0] = 1
-        dp[1] = if (s[0] == '0') 0 else 1
-
-        for (i in 2..n) {
-            val curr = s[i-1] - '0'
-            val prev = s[i-2] - '0'
-            val num = 10*prev + curr
-
-            if (curr > 0)
-                dp[i] += dp[i-1]
-
-            if (num >= 10 && num <= 26)
-                dp[i] += dp[i-2]
+    // Time : O(n) | Space : O(n)
+    fun numDecodings(string: String): Int {
+        if (string.length == 1) return if (string.last() != '0') 1 else 0
+        // dp array
+        val numberOfDecodings = IntArray(string.length + 1).apply {
+            this[lastIndex - 1] = if (string.last() == '0') 0 else 1
+            this[lastIndex] = if ("${string[string.lastIndex - 1]}${string[string.lastIndex]}".toInt() <= 26) 1 else 0
         }
-
-        return dp[n]
+        for (i in (string.lastIndex - 1) downTo 0) {
+            if (string[i] == '0') {
+                numberOfDecodings[i] = 0
+                continue
+            }
+            numberOfDecodings[i] += numberOfDecodings[i + 1]
+            numberOfDecodings[i] += numberOfDecodings[i + 2].takeIf {
+                (string[i] == '1' && string[i + 1] in '0'..'9') || (string[i] == '2' && string[i + 1] in '0'..'6')
+            } ?: 0
+        }
+        return numberOfDecodings[0]
     }
 }
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