solve: decode ways

decode-ways/wogha95.js

@@ -0,0 +1,48 @@
+// TC: O(N)
+// SC: O(N)
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numDecodings = function (s) {
+  if (s[0] === "0") {
+    return 0;
+  }
+  if (s.length === 1) {
+    return 1;
+  }
+
+  const dpTable = new Array(s.length).fill(0);
+  if (s[0] !== "0") {
+    dpTable[0] = 1;
+  }
+  if (s[1] !== "0") {
+    dpTable[1] += 1;
+  }
+  if (isValid(`${s[0]}${s[1]}`)) {
+    dpTable[1] += 1;
+  }
+
+  for (let index = 2; index < s.length; index++) {
+    if (s[index] !== "0") {
+      dpTable[index] += dpTable[index - 1];
+    }
+    if (s[index - 1] !== "0" && isValid(`${s[index - 1]}${s[index]}`)) {
+      dpTable[index] += dpTable[index - 2];
+    }
+  }
+
+  return dpTable[dpTable.length - 1];
+
+  function isValid(stringNumber) {
+    const number = Number(stringNumber);
+    if (number <= 0) {
+      return false;
+    }
+    if (27 <= number) {
+      return false;
+    }
+    return true;
+  }
+};