Java for Leetcode 144 Binary Tree Preorder traversal

Given a binary tree, return the preorder traversal of its nodes ' values.

For example:
Given binary Tree {1,#,2,3} ,

   1         2    /   3

Return [1,2,3] .

Binary tree pre-sequence traversal, root node → left subtree → right sub-tree

Problem solving idea One:

Recursive implementation, Java implementation is as follows:

    Public list<integer> preordertraversal (TreeNode root) {list<integer> List = new Arraylist<integer> () ; if (root = null) return List;list.add (Root.val); List.addall (Preordertraversal (Root.left)); List.addall ( Preordertraversal (Root.right)); return list;    }

Two ways to solve problems:

Using the stack implementation, the Java implementation is as follows:

Public list<integer> preordertraversal (TreeNode root) {list<integer> List = new Arraylist<integer> () ; if (root = null) return list; stack<treenode> stack = new stack<treenode> (); Stack.push (root); TreeNode pop = Root;while (!stack.isempty ()) {pop = Stack.pop (); List.add (pop.val); if (pop.right! = null) Stack.add ( Pop.right); if (pop.left! = null) Stack.add (pop.left);} return list;}

Java for Leetcode 144 Binary Tree Preorder traversal