LeetCode_Binary Tree Inorder Traversal

Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.
（中序遍历二叉树）

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

Example:

1. 递归

具体实现方法如下：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

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

2. 迭代 & 栈

不断将左子树加入到栈中，然后访问依次访问，最后再访问右子树。具体实现方法如下：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:   
        if not root:
            return []
        
        results, stack = [], []
        p = root
        while stack or p:
            while p:
                stack.append(p)
                p = p.left
            p = stack.pop()
            results.append(p.val)
            p = p.right
            
        return results