title
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?
solution
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
import java.util.*;
public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayDeque<TreeNode> stack=new ArrayDeque<>();
if(root==null) return new ArrayList<>();
TreeNode p=root;
ArrayList<Integer> res=new ArrayList<>();
while (p!=null||!stack.isEmpty()){
while (p!=null){
res.add(p.val);
stack.push(p);
p=p.left;
}
if(!stack.isEmpty()){
p=stack.peek().right;
stack.pop();
}
// System.out.print("1");
}
return res;
}
}