力扣——二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。

 示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        List<Integer> res = new ArrayList<>();
        while(p!=null || !stack.empty()){
            if(p==null){
                p = stack.pop();
            }
            res.add(p.val);
            if(p.right!=null){
                stack.push(p.right);
            }
            p = p.left;
            
        }
        
        return res;
    }
}