[DP] Optimize code style for Decode Ways

Author: Yi Gu

DP/DecodeWays.swift

@@ -8,41 +8,35 @@
 
 class DecodeWays {
     func numDecodings(_ s: String) -> Int {
-        let chars = Array(s.characters), len = chars.count
-        
-        guard len > 0 else {
-            return 0
-        }
-        
+        let sChars = Array(s.characters), len = sChars.count
         var dp = Array(repeating: 0, count: len + 1)
         dp[0] = 1
-        dp[1] = isValid(String(chars[0 ..< 1])) ? 1 : 0
-       
-        guard len >= 2 else {
-            return dp[len]
+        
+        guard len >= 1 else {
+            return 0
         }
 
-        for i in 2 ... len {
-            if isValid(String(chars[i - 1 ..< i])) {
+        for i in 1 ... len {
+            if isValid(String(sChars[i - 1 ..< i])) {
                 dp[i] += dp[i - 1]
             }
-            if isValid(String(chars[i - 2 ..< i])) {
+            if i >= 2 && isValid(String(sChars[i - 2 ..< i])) {
                 dp[i] += dp[i - 2]
             }
         }
 
         return dp[len]
     }
 
-    private func isValid(_ str: String) -> Bool {
-        let chars = Array(str.characters)
-        
-        if chars[0] == "0" {
+    private func isValid(_ numStr: String) -> Bool {
+        if Array(numStr.characters).first == "0" {
+            return false
+        }
+    
+        guard let num = Int(numStr) else {
             return false
         }
 
-        let num = Int(str)!
-        
-        return num > 0 && num <= 26
+        return num >= 1 && num <= 26
     }
 }