add Decode Ways solution

Author: HoonDongKang

decode-ways/HoonDongKang.ts

@@ -0,0 +1,51 @@
+/**
+ * [Problem]: [91] Decode Ways
+ *
+ * (https://leetcode.com/problems/decode-ways/description/)
+ */
+function numDecodings(s: string): number {
+    //시간복잡도 O(n)
+    //공간복잡도 O(n)
+    function memoizationFunc(s: string): number {
+        const memoization: Record<number, number> = {};
+
+        function dfs(index: number): number {
+            if (index in memoization) return memoization[index];
+            if (index === s.length) return 1;
+            if (s[index] === "0") return 0;
+
+            let result = dfs(index + 1);
+            if (index + 1 < s.length && +s.slice(index, index + 2) <= 26) {
+                result += dfs(index + 2);
+            }
+
+            memoization[index] = result;
+            return result;
+        }
+
+        return dfs(0);
+    }
+
+    //시간복잡도 O(n)
+    //공간복잡도 O(1)
+    function optimizedFunc(s: string): number {
+        let prev2 = 1;
+        let prev1 = s[0] === "0" ? 0 : 1;
+
+        for (let i = 1; i < s.length; i++) {
+            let curr = 0;
+
+            const one = +s.slice(i, i + 1);
+            const two = +s.slice(i - 1, i + 1);
+
+            if (one >= 1 && one <= 9) curr += prev1;
+            if (two >= 10 && two <= 26) curr += prev2;
+
+            prev2 = prev1;
+            prev1 = curr;
+        }
+
+        return prev1;
+    }
+    return optimizedFunc(s);
+}