每次查找第一个降序的首字符，如果不存在就删除结尾字符，链表，O(n)。

# include 
     
      # include 
      
       using namespace std;struct node{    char ch;    node *pre, *next;};void b(node *head, char *s){    head->ch = s[0];    node *tmp = head;    for (int i = 0; s[i]; ++i)    {        node *cur = new node;        cur->ch = s[i+1];        tmp->next = cur;        cur->pre  = tmp;        cur->next = NULL;        tmp = cur;    }}node* d(node *head, int n){    node *tmp = head;    while (n)    {        if ((tmp->ch) > (tmp->next->ch))        {            --n;            if (tmp == head)            {                head = head->next;                head->pre = NULL;                delete tmp;                tmp = head;            }            else            {                node *cur = tmp->pre;                tmp->pre->next = tmp->next;                tmp->next->pre = tmp->pre;                delete tmp;                tmp = cur;            }        }        else tmp = tmp->next;    }    return head;}void p(node *head){    node *cur = head;    while (cur->ch == '0') cur = cur->next;    if (cur->ch == '\0') puts("0");    else    {        while (cur->ch != '\0')        {            putchar(cur->ch);            cur = cur->next;        }        putchar('\n');    }}void k(node *p){    if (p->next)    {        k(p->next);        delete p;    }}int ss;char nn[2005];void solve(void){    node *head = new node;    head->pre = NULL;    head->next = NULL;    b(head, nn);    head = d(head, ss);    p(head);    k(head);}int main(){        while (~scanf("%s%d", nn, &ss))        solve();        return 0;}