二叉树：二叉树是每个结点最多有两个子树的树结构。通常子树被称作“左子树”和“右子树”
**

二叉树.png

python代码实现如下：

• 二叉树的创建
• 二叉树的层次遍历
• 二叉树的先序遍历
• 二叉树的中序遍历
• 二叉树的后序遍历

#coding:utf-8
class Node:
    def __init__(self, val=None, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

class Tree:
    def __init__(self, root=None):
        self.root = root

    def add(self, val):
        """
        添加节点元素
        :param val:
        :return:
        """
        node = Node(val)
        if self.root is None:
            self.root = node
            return
        queue = [self.root]
        while queue:
            cur = queue.pop(0)
            if cur.left is not None:
                queue.append(cur.left)
            else:
                cur.left = node
                return
            if cur.right is not None:
                queue.append(cur.right)
            else:
                cur.right = node
                return

    def breadth_travel(self):
        """
        层次遍历/广度优先
        :param val:
        :return:
        """
        if self.root is None:
            return
        queue = [self.root]
        while queue:
            cur = queue.pop(0)
            print(cur.val, end='')
            if cur.left is not None:
                queue.append(cur.left)
            if cur.right is not None:
                queue.append(cur.right)

    def left_travel(self, root):
        """
        前序遍历/深度优先
        :param val:
        :return:
        """
        if root is None:
            return
        print(root.val, end='')
        self.left_travel(root.left)
        self.left_travel(root.right)

    def mid_travel(self, root):
        """
        中序遍历/深度优先
        :param val:
        :return:
        """
        if root is None:
            return
        self.mid_travel(root.left)
        print(root.val, end='')
        self.mid_travel(root.right)

    def right_travel(self, root):
        """
        后序遍历/深度优先
        :param val:
        :return:
        """
        if root is None:
            return
        self.right_travel(root.left)
        self.right_travel(root.right)
        print(root.val, end='')

if __name__ == '__main__':
    tree = Tree()
    tree.add(0)
    tree.add(1)
    tree.add(2)
    tree.add(3)
    tree.add(4)
    tree.add(5)
    tree.add(6)
    tree.add(7)
    tree.add(8)
    tree.add(9)
    tree.breadth_travel()
    tree.left_travel(tree.root)
    tree.right_travel(tree.root)
    tree.mid_travel(tree.root)