Decode Ways

Author: Sunjae95

decode-ways/sunjae95.js

@@ -0,0 +1,76 @@
+/**
+ * @description
+ * brainstorming:
+ * 1. dp -> dp[i] = dp[i-1] + count
+ * 2. recursive function
+ *
+ * strategy:
+ * https://www.algodale.com/problems/decode-ways/
+ *
+ * result:
+ * 1. couldn't think of the conditions
+ *      true: 1~9, 10~27
+ *      false: 0, 0N, 28↑
+ * 2. persist solution that is top down
+ */
+
+// https://www.algodale.com/problems/decode-ways/ Solve 1
+/**
+ * time complexity: O(2^n)
+ * space complexity: O(n)
+ */
+var numDecodings = function (s) {
+  const search = (start) => {
+    if (start === s.length) return 1;
+    if (s[start] === "0") return 0;
+    if (start + 1 < s.length && Number(`${s[start]}${s[start + 1]}`) < 27) {
+      return search(start + 1) + search(start + 2);
+    }
+    return search(start + 1);
+  };
+
+  return search(0);
+};
+
+// https://www.algodale.com/problems/decode-ways/ Solve 2
+/**
+ * time complexity: O(2^n)
+ * space complexity: O(n)
+ */
+var numDecodings = function (s) {
+  const memo = new Map();
+  memo.set(s.length, 1);
+
+  const search = (start) => {
+    if (!!memo.get(start)) return memo.get(start);
+
+    if (s[start] === "0") memo.set(start, 0);
+    else if (start + 1 < s.length && Number(`${s[start]}${s[start + 1]}`) < 27)
+      memo.set(start, search(start + 1) + search(start + 2));
+    else memo.set(start, search(start + 1));
+
+    return memo.get(start);
+  };
+
+  return search(0);
+};
+
+// https://www.algodale.com/problems/decode-ways/ Solve 3
+/**
+ * time complexity: O(n)
+ * space complexity: O(n)
+ */
+var numDecodings = function (s) {
+  const dp = Array.from({ length: s.length + 1 }, (_, i) =>
+    i === s.length ? 1 : 0
+  );
+
+  for (let i = s.length - 1; i >= 0; i--) {
+    if (s[i] === "0") dp[i] = 0;
+    else if (i + 1 < s.length && Number(`${s[i]}${s[i + 1]}`) < 27)
+      dp[i] = dp[i + 1] + dp[i + 2];
+    else dp[i] = dp[i + 1];
+  }
+
+  return dp[0];
+};