请帮我写一下人狼羊菜过河问题的代码

好的，以下是人狼羊菜过河问题的 Python 代码：

def is_safe(state):
    # 判断当前状态是否安全
    if state[0] != state[1] and state[0] == state[2]:
        # 羊被狼吃了
        return False
    if state[1] != state[2] and state[1] == state[3]:
        # 菜被羊吃了
        return False
    return True

def is_goal(state):
    # 判断是否达到目标状态
    return state == (1, 1, 1, 1)

def get_next_states(state):
    # 获取下一步所有可能的状态
    next_states = []
    for i in range(4):
        # 考虑将第 i 个元素从对岸带到此岸
        if state[i] == state[0]:
            # 人和物品在同一岸上才能带
            new_state = list(state)
            new_state[i] = 1 - new_state[i]
            if is_safe(new_state):
                next_states.append(tuple(new_state))
    return next_states

def dfs(state, visited):
    # 深度优先搜索
    visited.add(state)
    if is_goal(state):
        return [state]
    for next_state in get_next_states(state):
        if next_state not in visited:
            path = dfs(next_state, visited)
            if path:
                return [state] + path
    return None

def solve():
    # 解决问题
    initial_state = (0, 0, 0, 0)
    visited = set()
    path = dfs(initial_state, visited)
    if path:
        print('成功通过河流！')
        for state in path:
            print(state)
    else:
        print('无解')

# 测试
solve()

程序中，`is_safe` 函数用于判断当前状态是否安全，`is_goal` 函数用于判断是否达到目标状态，`get_next_states` 函数用于获取下一步所有可能的状态，`dfs` 函数用于进行深度优先搜索，`solve` 函数用于解决问题。程序中使用集合 `visited` 用于记录已经访问过的状态，避免重复搜索。