"Leetcode-Interview algorithm classic-java implementation" "144-binary Tree Preorder Traversal (binary non-recursive pre-sequence traversal)"

"144-binary Tree Preorder Traversal (binary tree non-recursive pre-sequence traversal)" "leetcode-Interview algorithm classic-java Implementation" "All topics Directory Index" Original Question

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] .
Note:recursive solution is trivial, could do it iteratively?

Main Topic

Given a binary tree, output the results of the pre-order traversal, and try to use two methods to implement

Thinking of solving problems

The first: use recursion.
The second type: Using non-recursive methods

Code Implementation

Node class

publicclass TreeNode {    int val;    TreeNode left;    TreeNode right;    TreeNode(int x) { val = x; }}

The first method: the implementation class of the algorithm

ImportJava.util.LinkedList;ImportJava.util.List; Public  class solution {    Privatelist<integer> result; PublicList<integer>Preordertraversal(TreeNode Root) {result =NewLinkedlist<> (); Preorder (root);returnResult }Private void Preorder(TreeNode Root) {if(Root! =NULL) {Result.add (root.val);            Preorder (Root.left);        Preorder (root.right); }    }}

The second method: the implementation class of the algorithm

 PublicClass Solution { Public List<Integer>Preordertraversal (TreeNode root) {List<Integer>Result= NewLinkedList<>();if(Root!= NULL) {Deque<TreeNode> Stack = NewLinkedList<>();Stack.Add (root); while(!Stack.IsEmpty ()) {TreeNode node= Stack.Removelast (); Result.Add (node.Val);if(node.Right!= NULL) {Stack.Add (node.right); }if(node.Left!= NULL) {Stack.Add (node.left); }            }        }returnResult }}

Evaluation Results

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The result of the first method:

The result of the first method:

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"Leetcode-Interview algorithm classic-java implementation" "144-binary Tree Preorder Traversal (binary non-recursive pre-sequence traversal)"