solve decode ways

Author: grapefruit13

decode-ways/grapefruit13.ts

@@ -0,0 +1,23 @@
+function numDecodings(s: string): number {
+    if (s[0]==="0") return 0;
+
+    const dp = new Array(s.length).fill(0);
+    dp[0] = 1;
+
+    for (let i=1; i<s.length; i++) {
+        if (s[i] !== "0") {
+            dp[i] += dp[i-1];
+        }
+
+        const two = Number(s[i-1] + s[i]);
+
+        if (10 <= two && two <= 26) {
+            if (i === 1) {
+                dp[i] += 1;
+            } else {
+                dp[i] += dp[i-2];
+            }
+        }
+    }
+    return dp[s.length - 1];
+};