Print binary tree extreme corner in alternate order - being an interviewer

PrintBinaryTreeExtremeCornerInAlternateOrder.py

 //  Binary Tree 
#   
# //   Given a binary tree, Print nodes of extreme corners of each level but in alternate order.
# 
# //     3
# //    / \
# //   9  20
# //      /  \
# //    15   7
# 
# // print 3 20 15
# //  left most on the 0-th level
# // right most on the 1-th level
# // left most on the 2-th level. …..
# 
class TreeNode {
    TreeNode left;
    TreeNode right;
    int val;
}

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.right = None
        self.left = None

def zigzag_traversal(root):
    queue = []
    queue.append(root)
    flag = False
    if not root:
        return 
    while queue:
        next_level = []
        l = []
        for node in queue: 
            l.append(node.val) #[15, 7]
        
            if node.left:
                next_level.append(node.left.val) #[15,7]
            if node.right:
                next_level.append(node.right.val) #[9,20]
                
        if flag:
            print(l[0]) # print 3, 20, 15
        else:
            print(l[-1]) # print 20 # negative 1 is the last element of a list in python
        flag = not Flag #set Flag = False 
        queue = next_level # queue becomes [15, 7]