二叉树遍历-前序|中序|后序|层次 python实现

1、概念

四种遍历的基本思想：

前序遍历：根结点 ---> 左子树 ---> 右子树

中序遍历：左子树 ---> 根结点 ---> 右子树

后序遍历：左子树 ---> 右子树 ---> 根结点

层次遍历：从根结点开始，从左到右，按层次遍历就可以

2、四种遍历示例

前序遍历：1  2  4  5  7  8  3  6
中序遍历：4  2  7  5  8  1  3  6
后序遍历：4  7  8  5  2  6  3  1
层次遍历：1  2  3  4  5  6  7  8

3、python实现

class BinaryTree:
    def __init__(self,root):
        self.val = root
        self.left = None
        self.right = None
    def insertLeft(self, newNode):
        if self.left == None:
            self.left = BinaryTree(newNode)
        else:
            t = BinaryTree(newNode)
            t.left = self.left
            self.left = t
    def insertRight(self, newNode):
        if self.right == None:
            self.right = BinaryTree(newNode)
        else:
            t = BinaryTree(newNode)
            t.right = self.right
            self.right = t
    # 前序遍历
    def PreOrder(self, node):
        if node:
            print(node.val)
            self.PreOrder(node.left)
            self.PreOrder(node.right)
    # 中序遍历
    def InOrder(self, node):
        if node:
            self.InOrder(node.left)
            print(node.val)
            self.InOrder(node.right)
    # 后序遍历
    def PostOrder(self, node):
        if node:
            self.PostOrder(node.left)
            self.PostOrder(node.right)
            print(node.val)
    # 层次遍历
    def LevelOrder(self, node):
        if node == None:
            return
        stack = []
        stack.append(node)
        while stack!=[]:
            node = stack[0]
            if node.left:
                stack.append(node.left)
            if node.right:
                stack.append(node.right)
            print(node.val)
            stack.pop(0)

代码示例：

root = BinaryTree(18)
root.left = BinaryTree(7)
root.right = BinaryTree(11)
root.left.left = BinaryTree(3)
root.left.right = BinaryTree(4)
root.right.left = BinaryTree(5)
root.right.right = BinaryTree(6)
root.right.left.left = BinaryTree(1)
root.right.left.right = BinaryTree(3)
root.right.right.left = BinaryTree(2)
root.right.right.right = BinaryTree(4)

前序遍历：ouput: 18 7 3 4 11 5 1 3 6 2 4

中序遍历：output：3 7 4 18 1 5 3 11 2 6 4

后序遍历：output:3 4 7 1 3 5 2 4 6 11 18

层次遍历：output: 18 7 11 3 4 5 6 1 3 2 4

备注：原文出处 https://blog.csdn.net/Rai_Code/article/details/100006080