feat: 91. Decode Ways

decode-ways/HC-kang.ts

@@ -0,0 +1,31 @@
+// T.C: O(n)
+// S.C: O(n)
+function numDecodings(s: string): number {
+  const NUM_OF_ALPHA = 26;
+  const memo = new Map<number, number>();
+
+  function dfs(idx: number): number {
+    if (idx === s.length) {
+      return 1;
+    }
+    if (s[idx] === '0') {
+      return 0;
+    }
+    if (memo.has(idx)) {
+      return memo.get(idx)!;
+    }
+
+    let count = dfs(idx + 1);
+    if (
+      idx + 2 <= s.length && // check if idx + 2 is in the range
+      parseInt(s.slice(idx, idx + 2), 10) <= NUM_OF_ALPHA
+    ) {
+      count += dfs(idx + 2);
+    }
+
+    memo.set(idx, count);
+    return count;
+  }
+
+  return dfs(0);
+}