Binary Tree Level Order Traversal. Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

binary_tree_level_order_traversal.py

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""
import collections


class Solution:
    """
    @param root: The root of binary tree.
    @return: Level order in a list of lists of integers
    """
    def levelOrder(self, root):
        if not root:
            return []

        # # BFS
        # # 2 queues
        q1 = []
        q1.append(root)
        q2 = []

        ret = []
        while q1:
            lvl = []
            while q1:
                node = q1.pop(0)
                if node.left:
                    q2.append(node.left)
                if node.right:
                    q2.append(node.right)
                lvl.append(node.val)
            q1 = q2
            q2 = []
            ret.append(lvl)
        return ret

class Solution1:
    """
    @param root: The root of binary tree.
    @return: Level order in a list of lists of integers
    """
    def levelOrder(self, root):
        if not root:
            return []
        # # BFS
        # # 1 queue
        queue = collections.deque()
        queue.append(root)
        ret = []
        while queue:
            size = len(queue)
            lvl = []
            for _ in xrange(size):
                node = queue.popleft()
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
                lvl.append(node.val)
            ret.append(lvl)
        return ret
        

class Solution2:
    """
    @param root: The root of binary tree.
    @return: Level order in a list of lists of integers
    """
    def levelOrder(self, root):
        if not root:
            return []
        # DFS
        lvls = collections.defaultdict(list)
        self.dfs(root, 0, lvls)

        ret = []
        for _, values in sorted(lvls.items()):
            ret.append(values)
        return ret

    def dfs(self, root, cur_lvl, lvls):
        if root:
            lvls[cur_lvl].append(root.val)
            self.dfs(root.left, cur_lvl + 1, lvls)
            self.dfs(root.right, cur_lvl + 1, lvls)