43. Leetcode 94. 二叉树的中序遍历 (二叉树-二叉树遍历)

给定一个二叉树的根节点 root ，返回它的 中序 遍历。

 

示例 1：


输入：root = [1,null,2,3]
输出：[1,3,2]
示例 2：

输入：root = []
输出：[]
示例 3：

输入：root = [1]
输出：[1]
示例 4：


输入：root = [1,2]
输出：[2,1]
示例 5：


输入：root = [1,null,2]
输出：[1,2]


# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        # 方法一 递归
        if root == None:
            return []
        output = []
        output += self.inorderTraversal(root.left)
        output.append(root.val)
        output += self.inorderTraversal(root.right)

        return output

        # 方法二 迭代
        if root == None:
            return []
        stack = []
        output = []
        p_node = root
        while stack or p_node:
            while p_node:
                stack.append(p_node)
                p_node = p_node.left
            
            cur_node = stack.pop()
            output.append(cur_node.val)

            if cur_node.right:
                p_node = cur_node.right

        return output

        # 方法三 颜色标记法
        if root == None:
            return []
        WHITE, GARY = 0, 1
        stack = [(WHITE, root)]
        output = []

        while stack:
            color, root = stack.pop()
            if root != None:
                if color == WHITE:
                    stack.append((WHITE, root.right))
                    stack.append((GARY, root))
                    stack.append((WHITE, root.left))
                else:
                    output.append(root.val)

        return output