Python bigO question

def solution(number, k):
    num_s = str(number)
    answer = ''
    if k == 0:
        return number
    list1 = []
    list_a = []
    list_temp = []
    for i in range(len(str(number))):
        list1.append(int(num_s[i]))
    num1 = len(list1) - k

    while True:
        max_num = 0
        if len(list_a) == num1:
            answer = ''.join(map(str,list_a))
            return answer
        if k == 0:
            list_a = list_a + list1
            answer = ''.join(map(str,list_a))
            return answer

        for i in range(k+1):
            if list1[i] == 9:
                max_num = 9
            elif list1[i] >= max_num:
                max_num = list1[i]
        list_a.append(max_num)
        temp = list1.index(max_num)
        k = k - temp
        list1 = list1[temp+1:]