Update 91. Decode Ways.md

Author: TBXark

91. Decode Ways.md

@@ -31,7 +31,7 @@ Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
 class Solution {
     private var cache = [Int]()
     private func numDecodingsForSubString(start: Int, end: Int, c: [Int8]) -> Int {
-        let index = start * 2 + (end - start)
+        let index = start * 2 + (end - start) - 1
         if cache[index] >= 0  {
             return cache[index]
         }
@@ -59,7 +59,7 @@ class Solution {
     func numDecodings(_ s: String) -> Int {
         var c = s.utf8CString.map({  $0 - 48 })
         c.removeLast()
-        cache = [Int].init(repeating: -1, count: c.count * 2 + 2)
+        cache = [Int].init(repeating: -1, count: c.count * 2)
         return numDecodingsForSubString(start: 0, end: 1, c: c) + numDecodingsForSubString(start: 0, end: 2, c: c)
     }
 }