Binary Tree preorder iterator
/**
* 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> res=new ArrayList<>();
if(root==null) return res;
Stack<TreeNode> stack=new Stack<>();
stack.push(root);
while(!stack.isEmpty()){
TreeNode cur=stack.pop();
res.add(cur.val);
if(cur.right!=null) stack.push(cur.right);
if(cur.left!=null) stack.push(cur.left);
}
return res;
}
}
