tree related problems (update continuously)

leetcode Binary Tree Level Order Traversal

这道题是要进行二叉树的层次遍历，对于层次遍历，最简单直观的办法就是进行BFS。于是我们只需要维护一个队列就可以了，队列里面的元素需要记录该节点的内容和节点所在的层，依次从队列中取出节点进行扩展就可以了。

# Definition for a  binary tree node# class TreeNode:#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution:    # @param root, a tree node    # @return a list of lists of integers    def levelOrder(self, root):        if(root == None):            return []                queue = []        queue.append((0, root))                ans = []        cur = 0        cur_nodes = []        while len(queue) != 0:            level, fa = queue.pop(0)            if(level == cur):                cur_nodes.append(fa.val)            else:                ans.append(cur_nodes)                cur_nodes = []                cur_nodes.append(fa.val)                cur = level            if fa.left != None:                queue.append((level+1, fa.left))            if fa.right != None:                queue.append((level+1, fa.right))        ans.append(cur_nodes)        return ans