102.binary-tree-level-order-traversal.python3.py

#
# [102] Binary Tree Level Order Traversal
#
# https://leetcode.com/problems/binary-tree-level-order-traversal/description/
#
# algorithms
# Medium (44.45%)
# Total Accepted:    285.8K
# Total Submissions: 635.7K
# Testcase Example:  '[3,9,20,null,null,15,7]'
#
# Given a binary tree, return the level order traversal of its nodes' values.
# (ie, from left to right, level by level).
# 
# 
# For example:
# Given binary tree [3,9,20,null,null,15,7],
# 
# ⁠   3
# ⁠  / \
# ⁠ 9  20
# ⁠   /  \
# ⁠  15   7
# 
# 
# 
# return its level order traversal as:
# 
# [
# ⁠ [3],
# ⁠ [9,20],
# ⁠ [15,7]
# ]
# 
# 
#
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        if not root:
            return res 
        
        queue = []
        queue.append(root)
        while queue:
            cur_level = len(queue)
            cur_level_item = []
            while cur_level:
                temp = queue.pop(0)
                cur_level_item.append(temp.val)
                if temp.left:
                    queue.append(temp.left)
                if temp.right:
                    queue.append(temp.right)
                cur_level -= 1
            res.append(cur_level_item)
        return res