Add decode ways solution

Author: hyunjung-choi

decode-ways/hyunjung-choi.kt

@@ -0,0 +1,27 @@
+class Solution {
+    fun numDecodings(s: String): Int {
+        if (s.isEmpty() || s[0] == '0') return 0
+
+        val n = s.length
+        val dp = IntArray(n + 1)
+
+        dp[0] = 1
+        dp[1] = 1
+
+        for (i in 2..n) {
+            val currentChar = s[i - 1]
+            val prevChar = s[i - 2]
+
+            if (currentChar != '0') {
+                dp[i] += dp[i - 1]
+            }
+
+            val twoDigit = (prevChar - '0') * 10 + (currentChar - '0')
+            if (twoDigit in 10..26) {
+                dp[i] += dp[i - 2]
+            }
+        }
+
+        return dp[n]
+    }
+}