Update 0606-construct-string-from-binary-tree.kt

Author: a93a

kotlin/0606-construct-string-from-binary-tree.kt

@@ -1,3 +1,4 @@
+// recursion
 class Solution {
     fun tree2str(root: TreeNode?): String {
         val res = StringBuilder()
@@ -22,3 +23,39 @@ class Solution {
         return res.toString()
     }
 }
+
+// iterative
+class Solution {
+    fun tree2str(root: TreeNode?): String {
+        root?: return ""
+
+        val res = StringBuilder()
+        val stack = LinkedList<TreeNode?>().apply { addLast(root) }
+        val visited = HashSet<TreeNode?>()
+
+        while (stack.isNotEmpty()) {
+            val cur = stack.peekLast()
+
+            if (cur in visited) {
+                stack.removeLast()
+                res.append(")")
+            } else {
+                visited.add(cur)
+                res.append("(")
+                res.append(cur?.`val`)
+
+                if (cur?.left == null && cur?.right != null)
+                    res.append("()")
+                
+                if (cur?.right != null)
+                    stack.addLast(cur?.right)
+                if (cur?.left != null)
+                    stack.addLast(cur?.left)
+            }
+        }
+
+        res.deleteCharAt(0)
+        res.deleteCharAt(res.lastIndex)
+        return res.toString()
+    }
+}