feat: Add solution for LeetCode problem 91

Author: WhiteHyun

decode-ways/WhiteHyun.swift

@@ -0,0 +1,50 @@
+//  
+//  91. Decode Ways
+//  https://leetcode.com/problems/decode-ways/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/07/14.
+//  
+
+class Solution {
+
+  // dp
+  func numDecodings(_ s: String) -> Int {
+    var (current, previous) = (1, 0)
+    let array = s.compactMap { Int(String($0)) }
+    for index in array.indices.reversed() {
+      if array[index] == 0 {
+        (current, previous) = (0, current)
+      } else if index + 1 < array.count, array[index] * 10 + array[index + 1] <= 26 {
+        (current, previous) = (current + previous, current)
+      } else {
+        previous = current
+      }
+    }
+
+    return current
+  }
+
+  // Memoization
+  func numDecodingsUsingMemoization(_ s: String) -> Int {
+    let messages = s.compactMap { Int(String($0)) }
+    var cache: [Int: Int] = [s.count: 1]
+
+    func dfs(_ index: Int) -> Int {
+      if let cached = cache[index] {
+        return cached
+      }
+
+      if messages[index] == 0 {
+        cache[index] = 0
+      } else if index + 1 < s.count && messages[index] * 10 + messages[index + 1] < 27 {
+        cache[index] = dfs(index + 1) + dfs(index + 2)
+      } else {
+        cache[index] = dfs(index + 1)
+      }
+      return cache[index]!
+    }
+
+    return dfs(0)
+  }
+}