144 Binary Tree Preorder Traversal – Medium
Problem:
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?
Thoughts:
It is trivial to use recursion approach to solve this problem.
Preorder is to do a root, left, right.
Always append when meets a root. Then go deeper to the left subtree if got one.
Using a Stack is very helpful, which guaranteed all elements in the subtree will be appended prior to the right child.
Solutions:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result= new LinkedList<Integer>();
Stack<TreeNode> s = new Stack<TreeNode>();
if (root == null) {
return result;
}
s.push(root);
while (s.size() > 0) {
TreeNode node = s.pop();
result.add(node.val);
if (node.right != null) {
s.push(node.right);
}
if (node.left != null) {
s.push(node.left);
}
}
return result;
}
}