Binary Tree Level Order Traversal

Problem Statement

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

LeetCode link

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -1000 <= Node.val <= 1000

Code

Python Code

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """

        if root:
            stack = [[root]]
            l = [[root.val]]
        else:
            return []

        while stack:
            ll = []
            sl = []
            nl = stack.pop()
            for n in nl:
                if n.left:
                    ll.append(n.left.val)
                    sl.append(n.left)
                if n.right:
                    ll.append(n.right.val)
                    sl.append(n.right)
            if sl:
                stack.append(sl)
            if ll:
                l.append(ll)
        return l