算法详解参考：https://blog.csdn.net/Monster_ii/article/details/82115772
测试用例二叉树：
前序遍历结果：5 3 2 4 7 6 8
中序遍历结果：2 3 4 5 6 7 8
后序遍历结果：2 4 3 6 8 7 5
层序遍历结果：5 3 7 2 4 6 8
代码
1、前序遍历（递归与非递归）
class TreeNode:
def __init__(self,val):
self.val = val
self.left = None
self.right = None
self.parent = None
# 递归
qianxu = []
def PreOrder(root):
if not root:
return None
qianxu.append(root.val)
PreOrder(root.left)
PreOrder(root.right)
# 非递归
def Pre_Order(root):
data = []
stack = []
cur = root
while cur or stack:
while cur: # 若当前节点不为空
data.append(cur.val) # 得到当前节点值
stack.append(cur) # 压入栈中
cur = cur.left # 去往左子树
top = stack.pop() # 来到这里，说明当前节点为空，也就是当前节点的父节点无左子树，取出栈顶元素也就是当前节点的父节点
cur = top.right # 往右子树的方向，若右子树为空则看栈是不是空，若栈不为空，再取出栈顶节点，访问其右子树
return data
if __name__ == '__main__':
a1 = TreeNode(5)
a2 = TreeNode(3)
a3 = TreeNode(7)
a4 = TreeNode(2)
a5 = TreeNode(4)
a6 = TreeNode(6)
a7 = TreeNode(8)
a1.left = a2
a1.right = a3
a2.left = a4
a2.right = a5
a3.left = a6
a3.right = a7
PreOrder(a1)
print(qianxu)
print(Pre_Order(a1))
2、中序遍历（递归与非递归）
class TreeNode:
def __init__(self,val):
self.val = val
self.left = None
self.right = None
self.parent = None
# 递归
zhongxu = []
def InOrder(root):
if not root:
return None
InOrder(root.left)
zhongxu.append(root.val)
InOrder(root.right)
# 非递归
def In_Order(root):
data = []
stack = []
cur = root
while cur or stack:
while cur:
stack.append(cur)
cur = cur.left
top = stack.pop()
data.append(top.val)
cur = top.right
return data
if __name__ == '__main__':
a1 = TreeNode(5)
a2 = TreeNode(3)
a3 = TreeNode(7)
a4 = TreeNode(2)
a5 = TreeNode(4)
a6 = TreeNode(6)
a7 = TreeNode(8)
a1.left = a2
a1.right = a3
a2.left = a4
a2.right = a5
a3.left = a6
a3.right = a7
InOrder(a1)
print(zhongxu)
print(In_Order(a1))
3、后序遍历（递归与非递归）
class TreeNode:
def __init__(self,val):
self.val = val
self.left = None
self.right = None
self.parent = None
# 递归
houxu = []
def PastOrder(root):
if not root:
return None
PastOrder(root.left)
PastOrder(root.right)
houxu.append(root.val)
# 非递归
def Past_Order(root):
data = []
stack = []
stack.append(root)
while stack:
node = stack.pop()
if not node:
continue
stack.append(node.left)
stack.append(node.right)
data.append(node.val)
return data[::-1]
if __name__ == '__main__':
a1 = TreeNode(5)
a2 = TreeNode(3)
a3 = TreeNode(7)
a4 = TreeNode(2)
a5 = TreeNode(4)
a6 = TreeNode(6)
a7 = TreeNode(8)
a1.left = a2
a1.right = a3
a2.left = a4
a2.right = a5
a3.left = a6
a3.right = a7
PastOrder(a1)
print(houxu)
print(Past_Order(a1))
4、层序遍历
class TreeNode:
def __init__(self,val):
self.val = val
self.left = None
self.right = None
self.parent = None
def Level_Order(root):
if not root:
return
data = []
queue = []
queue.append(root)
while queue: # 队列不为空，说明还没遍历完
front = queue.pop(0) # 队列先进先出
if front.left:
queue.append(front.left) # 进左子节点
if front.right:
queue.append(front.right) # 进有右子节点
data.append(front.val)
return data
if __name__ == '__main__':
a1 = TreeNode(5)
a2 = TreeNode(3)
a3 = TreeNode(7)
a4 = TreeNode(2)
a5 = TreeNode(4)
a6 = TreeNode(6)
a7 = TreeNode(8)
a1.left = a2
a1.right = a3
a2.left = a4
a2.right = a5
a3.left = a6
a3.right = a7
print(Level_Order(a1))
