Binary Tree Preorder Traversal

For example:

Given binary tree {1,#,2,3},

1
    \
     2
    /
   3

return [1,2,3].

分析：此题考查二叉树的前序遍历，此处使用非递归遍历方式实现

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        TreeNode p = root;

        //双端队列，实现栈的功能
        Deque<TreeNode> stack = new ArrayDeque<>();
        
        while(p!=null || !stack.isEmpty()){
            while(p!= null){
                list.add(p.val);
                stack.push(p);
                p = p.left;
            }
            
            if(!stack.isEmpty()){
                p = stack.pop();
                p = p.right;
            }
        }
        
        return list;
    }
}