python 合并列表中有相同元素的列表

输入列表： [[1,2,3], [2,1], [3,4], [5], [6,7]] ，把其中有相同值的列表进行合并
输出列表：[[1, 2, 3, 4], [5], [6, 7]]

代码：

def found_idx(lst, k):
    """
    lst: [[1,2,3], [2,1], [3,4], [5], [6,7]]
    k = 3
    return: [0,2], [1,2,3,4]  # idx, which '3' in [1,2,3], [3,4]
    """
    idx = []
    for i,js in enumerate(lst):
        for j in js:
            if j == k:
                idx.append(i)
                continue
    mg_lst = []
    for x in idx:
        mg_lst += lst[x]
    return idx, list(set(mg_lst))


def merge_lst(lst):
    """
    lst: [[1,2,3], [2,1], [3,4], [5], [6,7]]
    return: [[1,2,3,4], [5], [6,7]]
    """
    outlst = []
    for i, js in enumerate(lst):
        for j in js:
            idx, mg_lst = found_idx(lst, j)
            if mg_lst not in outlst:
                outlst.append(list(mg_lst))
    if outlst == lst:
        return outlst
    else:
        return merge_lst(outlst)

merge_lst([[1,2,3], [2,1], [3,4], [5], [6,7]])
# [[1, 2, 3, 4], [5], [6, 7]]