Binary Tree Preorder Traversal —— LeetCode

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?

题目大意：非递归方式实现二叉树的前序遍历。

解题思路：自己实现一个栈，循环（根节点直接入栈并且入res List，循环入栈左子树，pop栈，入栈右子树）。

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        while(curr!=null||!stack.isEmpty()){
            while(curr!=null){
                stack.push(curr);
                res.add(curr.val);
                curr=curr.left;
            }
            curr = stack.pop();
            curr = curr.right;
        }
        return res;
    }
}