[DP] Refactor solution to Decode Ways

Author: soapyigu

DP/DecodeWays.swift

@@ -8,37 +8,28 @@
 
 class DecodeWays {
     func numDecodings(_ s: String) -> Int {
-        let sChars = Array(s)
+        let s = Array(s)
         var dp = Array(repeating: 0, count: s.count + 1)
         dp[0] = 1
 
-        guard s.count >= 1 else {
-            return 0
-        }
-        
         for i in 1...s.count {
-            if String(sChars[i - 1..<i]).isValid {
+            if s[i - 1] != "0" {
                 dp[i] += dp[i - 1]
             }
-            if i >= 2 && String(sChars[i - 2..<i]).isValid {
+            
+            if i > 1 && isValid(s, i - 2, i - 1) {
                 dp[i] += dp[i - 2]
-            }
+            } 
         }
 
         return dp[s.count]
     }
-}
-
-extension String {
-    var isValid: Bool {
-        if let first = first, first == "0" {
-            return false
-        }
-        
-        guard let num = Int(self) else {
-            return false
+    
+    private func isValid(_ s: [Character], _ start: Int, _ end: Int) -> Bool {
+        guard let num = Int(String(s[start...end])) else {
+            fatalError()
         }
 
-        return 0 < num && 26 >= num
+        return num >= 10 && num <= 26
     }
 }

README.md

@@ -4,7 +4,7 @@
 ![Leetcode](./logo.png?style=centerme)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 316 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
+[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 318 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
 
 ## Contributors
 
@@ -226,7 +226,7 @@
 [Nested List Weight Sum II](https://leetcode.com/problems/nested-list-weight-sum-ii/)| [Swift](./DP/NestedListWeightSumII.swift)| Medium| O(n)| O(n)|
 [Flip Game II](https://leetcode.com/problems/flip-game-ii/)| [Swift](./DP/FlipGameII.swift)| Medium| O(n)| O(n)|
 [Can I Win](https://leetcode.com/problems/can-i-win/)| [Swift](./DP/CanIWin.swift)| Medium| O(2^n)| O(n)|
-[Decode Ways](https://leetcode.com/problems/decode-ways/)| [Swift](./DP/DecodeWays.swift) | O(n)|O(n)|
+[Decode Ways](https://leetcode.com/problems/decode-ways/)| [Swift](./DP/DecodeWays.swift) | Medium| O(n)|O(n)|
 [Minimum Path Sum](https://leetcode.com/problems/minimum-path-sum/)| [Swift](./DP/MinimumPathSum.swift)| Medium| O(mn)| O(mn)|
 [Generate Parentheses](https://leetcode.com/problems/generate-parentheses/)| [Swift](./DP/GenerateParentheses.swift)| Medium| O(2^n)| O(n)|
 [Different Ways to Add Parentheses](https://leetcode.com/problems/different-ways-to-add-parentheses/)| [Swift](./DP/DifferentWaysAddParentheses.swift)| Medium| O(n^n)| O(n)|