left_visit.py

import unittest


class Node:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

    def __str__(self):
        return self.val


class NodeWrapper:
    def __init__(self, node, level):
        self.node = node
        self.level = level

    def __str__(self):
        return '{}-{}'.format(self.node, self.level)


def left_visit(root):
    res = []
    que = []
    lastLevel = 0
    que.append(NodeWrapper(root, lastLevel + 1))
    while que:
        nw = que.pop(0)
        if nw.level > lastLevel:
            res.append(nw.node.val)
            lastLevel = nw.level

        if nw.node.left:
            que.append(NodeWrapper(nw.node.left, lastLevel + 1))
        if nw.node.right:
            que.append(NodeWrapper(nw.node.right, lastLevel + 1))

    return res


class LeftVisitTest(unittest.TestCase):
    @staticmethod
    def create_tree():
        root = Node(1)
        root.left = Node(2)
        root.right = Node(3)
        root.left.left = Node(4)
        root.left.right = Node(5)
        root.right.left = Node(6)
        root.right.right = Node(7)
        root.right.right.right = Node(8)
        return root

    def setUp(self):
        self.root = LeftVisitTest.create_tree()

    def test_left_vist(self):
        res = [1, 2, 4, 8]
        left_visit_node = left_visit(self.root)
        self.assertListEqual(res, left_visit_node)
        print(left_visit_node)

    def tearDown(self):
        self.root = None


if __name__ == '__main__':
    unittest.main()

给定一棵二叉树，将其平放到桌面上，只打印从左侧看到的节点。
利用二叉树的层次遍历，从上到下，从左到右，每层只打印第一个节点。