寻找二叉树的叶子

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def find_leaves(root):
    leaves = []
    if root is None:
        return leaves
    if root.left is None and root.right is None:
        leaves.append(root.val)
    else:
        leaves.extend(find_leaves(root.left))
        leaves.extend(find_leaves(root.right))
    return leaves

# 示例用法
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
root.right.left = TreeNode(6)
root.right.right = TreeNode(7)

result = find_leaves(root)
print(result)  # 输出 [4, 5, 6, 7]