decode ways solution

Author: nowrobin

decode-ways/nowrobin.js

@@ -0,0 +1,30 @@
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numDecodings = function(s) {
+    if (!s || s[0] === '0') {
+        return 0;
+    }
+
+    const n = s.length;
+    const dp = new Array(n + 1).fill(0);
+    dp[0] = 1;
+    dp[1] = 1;
+
+    for (let i = 2; i <= n; ++i) {
+        const oneDigit = parseInt(s[i - 1]);
+        const twoDigits = parseInt(s.substring(i - 2, i));
+
+        if (oneDigit !== 0) {
+            dp[i] += dp[i - 1];
+        }
+
+        if (10 <= twoDigits && twoDigits <= 26) {
+            dp[i] += dp[i - 2];
+        }
+    }
+
+    return dp[n];
+};
+