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].
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
Stack<TreeNode> nodeStack=new Stack<TreeNode>();
List<Integer> list=new ArrayList<Integer>();
TreeNode node=root;
while(!nodeStack.isEmpty() || node!=null){
if(node!=null){
list.add(node.val);
if(node.right!=null){
nodeStack.add(node.right);
}
node=node.left;
}
else{
node=nodeStack.peek();
nodeStack.pop();
}
}
return list;
}
}