二叉树的遍历算法并测试
时间: 2023-11-28 16:04:57 浏览: 45
二叉树的遍历有三种方式:前序遍历、中序遍历和后序遍历。下面分别介绍三种遍历算法并附上 Python 代码和测试用例。
### 前序遍历
前序遍历即先访问根节点,再遍历左子树,最后遍历右子树。在代码实现中,可以使用递归或栈来实现。
```python
class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
def preorderTraversal(root: TreeNode) -> List[int]:
if root is None:
return []
res = []
stack = [root]
while stack:
node = stack.pop()
res.append(node.val)
if node.right:
stack.append(node.right)
if node.left:
stack.append(node.left)
return res
```
测试用例:
```python
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
assert preorderTraversal(root) == [1, 2, 4, 5, 3]
```
### 中序遍历
中序遍历即先遍历左子树,再访问根节点,最后遍历右子树。在代码实现中,可以使用递归或栈来实现。
```python
def inorderTraversal(root: TreeNode) -> List[int]:
if root is None:
return []
res = []
stack = []
node = root
while stack or node:
while node:
stack.append(node)
node = node.left
node = stack.pop()
res.append(node.val)
node = node.right
return res
```
测试用例:
```python
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
assert inorderTraversal(root) == [4, 2, 5, 1, 3]
```
### 后序遍历
后序遍历即先遍历左子树,再遍历右子树,最后访问根节点。在代码实现中,可以使用递归或栈来实现。
```python
def postorderTraversal(root: TreeNode) -> List[int]:
if root is None:
return []
res = []
stack = [root]
while stack:
node = stack.pop()
if node:
res.append(node.val)
stack.append(node.left)
stack.append(node.right)
return res[::-1]
```
测试用例:
```python
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
assert postorderTraversal(root) == [4, 5, 2, 3, 1]
```
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