Optimize solution to Decode String

iHackSubhodip · iHackSubhodip · commit e8c5a7b83d5a · 2019-04-10T13:38:34.000+05:30
diff --git a/README.md b/README.md
@@ -148,6 +148,7 @@
 [Binary Tree Preorder Traversal](https://leetcode.com/problems/binary-tree-preorder-traversal/)| [Swift](./Stack/PreorderTraversal.swift)| Medium| O(n)| O(n)|
 [Binary Tree Inorder Traversal](https://leetcode.com/problems/binary-tree-inorder-traversal/)| [Swift](./Stack/InorderTraversal.swift)| Medium| O(n)| O(n)|
 [Binary Tree Postorder Traversal](https://leetcode.com/problems/binary-tree-postorder-traversal/)| [Swift](./Stack/PostorderTraversal.swift)| Hard| O(n)| O(n)|
+[Decode String](https://leetcode.com/problems/decode-string/)| [Swift](./Stack/DecodeString.swift)| Medium| O(n)| O(n)|
 
 
 ## Tree
diff --git a/Stack/DecodeString.swift b/Stack/DecodeString.swift
@@ -0,0 +1,52 @@
+/**
+ * Question Link: https://leetcode.com/problems/decode-string/
+ * Primary idea: Primary idea is to maintain two stacks[i.e. countStack and characterStack].
+ *               Traverse the given string and process the elements into the respective stacks, and accordingly update the result.
+ * Time Complexity: O(n), Space Complexity: O(n)
+ * 
+ */
+
+
+class Solution {
+    func decodeString(_ s: String) -> String {
+        var result = ""
+        var countStack = [Int]()
+        var characterStack = [String]()
+        let allowedDigits = Set(["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"])
+        var arrayString = Array(s), i = 0
+        
+        while i < arrayString.count{
+            
+            if allowedDigits.contains(String(arrayString[i])){
+                
+                var count = 0
+                while allowedDigits.contains(String(arrayString[i])){
+                    count = 10 * count + Int(String(arrayString[i]))!
+                    i += 1
+                }
+                countStack.append(count)
+            }else if arrayString[i] == "["{
+                
+                characterStack.append(result)
+                result = ""
+                i += 1
+            }else if arrayString[i] == "]"{
+                
+                if var temp = characterStack.popLast(), let repeatTime = countStack.popLast(){
+                    
+                    for _ in 0..<repeatTime{
+                        temp.append(result)
+                    }
+                    result = temp
+                }
+                i += 1
+            }else{
+                
+                result.append(arrayString[i])
+                i += 1
+            }
+        }
+        
+        return result
+    }
+}
