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]
提示:
- 树中节点数目在范围 [0, 100] 内
- -100 <= Node.val <= 100
来源:力扣(LeetCode)
链接:https://leetcode-cn.com/problems/binary-tree-inorder-traversal
著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。
一解
使用递归:
# 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]:
self.res = []
def dfs(root):
if not root: return None
dfs(root.left)
self.res.append(root.val)
dfs(root.right)
dfs(root)
return self.res
- 时间复杂度:O(n)
- 空间复杂度:O(n)
二解
使用栈模拟递归,重复下列过程:
- 向栈中依次加入:根节点 → 左子节点 → 左子节点
- 出栈,并入栈“出栈节点”的“右子节点“
# 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]:
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
- 时间复杂度 O(n):n 为二叉树节点数量,二叉树每个节点都会被访问一次
- 空间复杂度 O(n):取决于栈深度,在树是一条链时达到最大
三解
Morris 遍历,此方法是树的遍历最优解!建议学会!
当前节点 cur ,一开始 cur 来到树头,循环下面过程,直到 cur = null
。
- cur 无左树,
cur = cur.right
- cur 有左树,找到左树的最右节点
most right
-
most right
的右指针指向 null ,most right.right = cur, cur = cur.left
-
most right
的右指针指向 cur ,most right.right = null, cur = cur.right
-
# 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]:
res = []
while root:
if root.left:
predecessor = root.left
while predecessor.right != None and predecessor.right != root:
predecessor = predecessor.right
if not predecessor.right:
predecessor.right = root
root = root.left
else:
res.append(root.val)
predecessor.right = None
root = root.right
else:
res.append(root.val)
root = root.right
return res
- 时间复杂度:O(n)
- 空间复杂度:O(1)