Create 91. Decode Ways.md

Author: TBXark

91. Decode Ways.md

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+# 91. Decode Ways
+
+### 2020-07-31
+
+A message containing letters from A-Z is being encoded to numbers using the following mapping:
+
+```
+'A' -> 1
+'B' -> 2
+...
+'Z' -> 26
+```
+Given a non-empty string containing only digits, determine the total number of ways to decode it.
+
+Example 1:
+```
+Input: "12"
+Output: 2
+Explanation: It could be decoded as "AB" (1 2) or "L" (12).
+```
+Example 2:
+```
+Input: "226"
+Output: 3
+Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
+```
+
+# Solution
+
+```swift
+class Solution {
+    private var cache = [Int]()
+    private func numDecodingsForSubString(start: Int, end: Int, c: [Int8]) -> Int {
+        let index = start * 2 + (end - start)
+        if cache[index] >= 0  {
+            return cache[index]
+        }
+        guard end <= c.count, c[start] != 0, isLetters(c[start..<end]) else {
+            cache[index] = 0
+            return 0
+        }
+        if end == c.count {
+            cache[index] = 1
+            return 1
+        }
+        let count = numDecodingsForSubString(start: end, end: end + 1, c: c) + numDecodingsForSubString(start: end, end: end + 2, c: c)
+        cache[index] = count
+        return count
+    }
+    
+    private func isLetters(_ array: ArraySlice<Int8>) -> Bool {
+        var i = Int8(0)
+        for c in array {
+            i = i * 10 + c
+        }
+        return i >= 1 && i <= 26
+    }
+
+    func numDecodings(_ s: String) -> Int {
+        var c = s.utf8CString.map({  $0 - 48 })
+        c.removeLast()
+        cache = [Int].init(repeating: -1, count: c.count * 2 + 2)
+        return numDecodingsForSubString(start: 0, end: 1, c: c) + numDecodingsForSubString(start: 0, end: 2, c: c)
+    }
+}
+
+
+```