bfs (easy)

队列特性：
时间复杂度：
bfs: 图的广度优先遍历

from collections import deque
def bfs(graph, root):
    queue = [root]
    seen = set([root])
    while len(queue):
        head = queue[0]
        # head = queue.popleft() 
        for i in graph[head]:
            if i not in seen:
                queue.append(i)
                seen.add(i)

二叉树的bfs:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        ans = []
        queue = [root]
        while queue:
            tmp = []
            l = len(queue)
            for i in range(l):
                node = queue.pop(0)
                tmp.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            ans.append(tmp)
        return ans