144. Binary Tree Preorder Traversal java solutions

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?

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/**
 * 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) {
        ArrayList<Integer> anslist = new ArrayList<Integer>();
        if(root == null) return anslist;
        Stack<TreeNode> s = new Stack<TreeNode>();
        s.push(root);
        while(!s.empty()){
            TreeNode n = s.pop();
            anslist.add(n.val);
            if(n.right != null) s.push(n.right);
            if(n.left != null) s.push(n.left);
        }
        return anslist;
    }
}

使用栈来模拟先序遍历。 递归的方法很简单，这里就不写了。