题意：让你输出前N个大写字母的前K个排列，按字典序，很水，直接dfs。

#include<cstdio>

#include<string>

#include<cstring>

#include<iostream>

#include<algorithm>

using namespace std;

int vis[26], N, K, cnt;

void dfs(int dep, string str){

    if(cnt == K) return;

    if(dep == N){

        cout << str << endl;

        cnt ++;

        return;

    }

    for(int i = 0;i < N;i ++){

        if(!vis[i]){

            vis[i] = 1;

            char ch = i + 'A';

            dfs(dep + 1, str + ch);

            if(cnt == K) return;

            vis[i] = 0;

        }

    }

}

int main(){

    int t,CASE(0);

    scanf("%d", &t);

    while(t--){

        cnt = 0;

        memset(vis, 0, sizeof vis);

        scanf("%d%d", &N, &K);

        printf("Case %d:\n", ++CASE);

        dfs(0, "");

    }

    return 0;

}