Daily LeetCode 94. Binary Tree Inorder Traversal

https://leetcode.com/problems/binary-tree-inorder-traversal/

Medium

问题描述：

Given a binary tree, return the inorder traversal of its nodes’ values.

Example:

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

Output: [1,3,2]

Follow up: Recursive solution is trivial, could you do it iteratively?

思路及代码：

这一题是要中序遍历二叉树

方法1:

采用递归的方式进行遍历

def inorderTraversal(root):
    def traversal(root, res):
        if root:
            traversal(root.left, res)
            res.append(root.val)
            traversal(root.right, res)

    res = []
    traversal(root, res)

    return res

方法2:

采用非递归的方式进行中序遍历

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return None

        stack = []
        res = []

        while root or stack:
            while root:
                stack.append(root)
                root = root.left
            root = stack.pop()
            res.append(root.val)
            root = root.right

        return res