Solution.java

/// Leetcode 144. Binary Tree Preorder Traversal
/// https://leetcode.com/problems/binary-tree-preorder-traversal/description/
///
/// 课程中在这里暂时没有介绍这个问题
/// 该代码主要用于使用Leetcode上的问题测试我们的BST类
/// 该测试主要测试前序遍历的非递归写法

import java.util.List;
import java.util.LinkedList;
import java.util.Stack;

public class Solution {

    // Definition for a binary tree node.
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public List<Integer> preorderTraversal(TreeNode root) {

        List<Integer> res = new LinkedList<>();
        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;
    }
}