solve: decode ways

decode-ways/tolluset.ts

@@ -0,0 +1,26 @@
+/**
+ * TC: O(n)
+ * SC: O(n)
+ */
+function numDecodings(s: string): number {
+  const n = s.length;
+  const dp = new Array(n + 1).fill(0);
+
+  dp[0] = 1;
+  dp[1] = Number(s[0]) === 0 ? 0 : 1;
+
+  for (let i = 2; i <= n; i++) {
+    const oneDigit = Number(s.slice(i - 1, i));
+    const twoDigits = Number(s.slice(i - 2, i));
+
+    if (oneDigit >= 1 && oneDigit <= 9) {
+      dp[i] += dp[i - 1];
+    }
+
+    if (twoDigits >= 10 && twoDigits <= 26) {
+      dp[i] += dp[i - 2];
+    }
+  }
+
+  return dp[n];
+}