二叉树的中序遍历

题目

给定一个二叉树的根节点root，返回它的中序遍历。

思路

递归实现很简单。

# 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]:
        # 左 根 右
        if root is None:
            return []
        return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)

迭代算法实现 重点重点

# 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]:
        # 左 根 右
        ans = []
        if root is None: return ans
        s = [] # 定义一个显式栈stack
        while root is not None or len(s) != 0:
            while root is not None:
                s.append(root)
                root = root.left
            root = s.pop()
            ans.append(root.val)
            root = root.right
        return ans