广度优先搜索BFS

def solve():
    from collections import deque
    class TreeNode:
        def __init__(____):
            self.val = val
            self.left = left
            self.right = right
    
    # 构建二叉树
    #     ____
    #    / \
    #   ____  ____
    #     /  \
    #    ____   ____
    root = TreeNode(____)
    root.left = TreeNode(____)
    root.right = TreeNode(____)
    root.right.left = TreeNode(____)
    root.right.right = TreeNode(____)
    
    result = []
    queue = deque(____)
    while queue:
        node = queue.popleft()
        result.append(str(____))
        if node.left:
            queue.append(____)
        if node.right:
            queue.append(____)
    
    print('____'.join(____))