Description
https://leetcode.com/problems/binary-tree-preorder-traversal/description/
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 you do it iteratively?
Explanation
To return the preorder traversal of tree nodes values iteratively, we introduce a stack. With a stack, we can easily iterative nodes with preorder (root, left, right) sequence.
Video Tutorial
Java Solution
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
if (root == null) {
return result;
}
stack.push(root);
while (!stack.empty()) {
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;
}
}