Add decode-ways in TypeScript

Author: Jeehay28

decode-ways/Jeehay28.ts

@@ -0,0 +1,117 @@
+// Approach 4:
+// Time Complexity: O(n)
+// ✅ Space Complexity: O(1)
+
+function numDecodings(s: string): number {
+  // s:   2 2 6
+  // dp:    ? ? 1
+  //       cur nxt
+
+  let next = 0,
+    current = 1;
+
+  for (let start = s.length - 1; start >= 0; start--) {
+    const temp = current;
+
+    if (s[start] === "0") {
+      // dp[start] = 0
+      current = 0;
+      next = temp;
+    } else if (
+      start + 1 < s.length &&
+      parseInt(s.substring(start, start + 2)) < 27
+    ) {
+      // dp[start] = dp[start + 1] + dp[start + 2]
+      current = current + next;
+      next = temp;
+    } else {
+      // dp[start] = dp[start + 1]
+      next = temp;
+    }
+  }
+  return current;
+}
+
+
+// Approach 3: Dynamic Programming
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+// function numDecodings(s: string): number {
+//   //    12
+//   // dp 001
+//   //    211
+
+//   //    226
+//   // dp 0001
+//   //    3211
+
+//   const dp = Array.from({ length: s.length + 1 }, (el) => 0);
+//   dp[dp.length - 1] = 1;
+
+//   for (let start = s.length - 1; start >= 0; start--) {
+//     if (s[start] === "0") {
+//       dp[start] = 0;
+//     } else if (
+//       start + 1 < s.length &&
+//       parseInt(s.substring(start, start + 2)) < 27
+//     ) {
+//       dp[start] = dp[start + 1] + dp[start + 2];
+//     } else {
+//       dp[start] = dp[start + 1];
+//     }
+//   }
+
+//   return dp[0];
+// }
+
+
+// Approach 2
+// ✅ Time Complexity: O(2^n) -> O(n)
+// Space Complexity: O(n)
+
+// function numDecodings(s: string): number {
+//   const memo = new Map<number, number>();
+//   memo.set(s.length, 1);
+
+//   const dfs = (start: number) => {
+//     if (memo.has(start)) return memo.get(start);
+//     if (s[start] === "0") {
+//       memo.set(start, 0);
+//     } else if (
+//       start + 1 < s.length &&
+//       parseInt(s.substring(start, start + 2)) < 27
+//     ) {
+//       memo.set(start, dfs(start + 1)! + dfs(start + 2)!);
+//     } else {
+//       memo.set(start, dfs(start + 1)!);
+//     }
+//     return memo.get(start);
+//   };
+//   return dfs(0)!;
+// }
+
+
+// Approach 1
+// ❌ Time Limit Exceeded!
+// Time Complexity: O(2^n), where n = s.length
+// Space Compexity: O(n), due to recursive call stack
+// function numDecodings(s: string): number {
+//   const dfs = (start: number) => {
+//     if (start === s.length) {
+//       return 1;
+//     }
+
+//     if (s[start] === "0") {
+//       return 0;
+//     }
+
+//     if (start + 1 < s.length && parseInt(s.substring(start, start + 2)) < 27) {
+//       return dfs(start + 1) + dfs(start + 2);
+//     } else {
+//       return dfs(start + 1);
+//     }
+//   };
+
+//   return dfs(0);
+// }