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

输出: [1,3,2]

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struct Solution;

// @lc code=start
// Definition for a binary tree node.
// #[derive(Debug, PartialEq, Eq)]
// pub struct TreeNode {
//   pub val: i32,
//   pub left: Option<Rc<RefCell<TreeNode>>>,
//   pub right: Option<Rc<RefCell<TreeNode>>>,
// }
//
// impl TreeNode {
//   #[inline]
//   pub fn new(val: i32) -> Self {
//     TreeNode {
//       val,
//       left: None,
//       right: None
//     }
//   }
// }
use std::cell::RefCell;
use std::rc::Rc;
impl Solution {
    /// ## 解题思路
    /// - 递归+深度遍历
    pub fn inorder_traversal1(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
        fn inorder_dfs(node: &Option<Rc<RefCell<TreeNode>>>, res: &mut Vec<i32>) {
            match node {
                None => {}
                Some(x) => {
                    inorder_dfs(&x.borrow().left, res);
                    res.push(x.borrow().val);
                    inorder_dfs(&x.borrow().right, res);
                }
            }
        }

        let mut res = vec![];
        inorder_dfs(&root, &mut res);

        res
    }
    /// - 栈
    pub fn inorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
        let mut res = vec![];
        let mut pending_nodes = vec![];
        let mut node = root;
        loop {
            // 将所有左子节点入栈
            while let Some(n) = node {
                node = n.borrow_mut().left.take();
                pending_nodes.push(n);
            }

            // 弹出栈顶节点
            if let Some(n) = pending_nodes.pop() {
                // 处理当前节点
                res.push(n.borrow().val);
                //
                node = n.borrow_mut().right.take();
            } else {
                break;
            }
        }

        res
    }
}
// @lc code=end

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class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        if not root:
            return res
        if root.left:
            res += self.inorderTraversal(root.left)
        res.append(root.val)
        if root.right:
            res += self.inorderTraversal(root.right)

        return res

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class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        if not root:
            return res

        pending = [] #
        cur = root
        while True:
            while cur: #pending current node if left exists
                pending.append(cur)
                cur = cur.left
            if not pending:
                return res
            cur = pending.pop()
            res.append(cur.val)
            cur = cur.right