Tree Traversal
- preorder traversal
- A -> B -> D -> H -> I -> E -> C -> F -> G
- inorder traversal
- H -> D -> I -> B -> E -> A -> F -> C -> G
- postorder traversal
- H -> I -> D -> E -> B -> F -> G -> C -> A
- Reference
- «https://m.blog.naver.com/PostView.naver?isHttpsRedirect=true&blogId=4717010&logNo=60209908735>
Example : preorder traversal
Problem
https://leetcode.com/problems/binary-tree-preorder-traversal/
Result
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Input: root = [1,null,2,3]
Output: [1,2,3]
Answer
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<Integer>();
if (root == null) return result;
Stack<TreeNode> stack = new Stack<TreeNode>();
stack.push(root);
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
result.add(node.val);
if (node.right != null) stack.push(node.right);
if (node.left != null) stack.push(node.left);
}
return result;
}
}
Example : postorder traversal
Problem
https://leetcode.com/problems/binary-tree-postorder-traversal/
Result
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2
Input: root = [1,null,2,3]
Output: [3,2,1]
Answer
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
LinkedList<Integer> result = new LinkedList<>();
if (root == null) return result;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
result.addFirst(node.val); // difference
if (node.left != null) stack.push(node.left); // difference
if (node.right != null) stack.push(node.right); // difference
}
return result;
}
}
- Reference :
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