CSU 1563 Lexicography（全排列第K大）

题意

输出字符串全排列第K大的数字。

AC代码

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
const int N = 100;
char str[N], ans[N];
ll factor[N], cnt[N];
void get_factor() { //计算阶乘
    factor[0] = 1;
    for(int i = 1; i <= 16; i++)
        factor[i] = factor[i-1] * i;
}
ll func(int sum) { //传入该可重集的个数，返回该可重集的全排列的个数
    ll ret = 1;
    for(int i = 0; i < 26; i++)
        ret *= factor[cnt[i]];
    return factor[sum] / ret;
}
ll tar;
bool dfs(int cur, int len) {
    if(cur == len) return true;
    for(int i = 0; i < 26; i++) {
        if(cnt[i]) {
            cnt[i]--;
            ll ret = func(len - cur - 1);
            cnt[i]++;
            if(ret < tar) tar -= ret;
            else {
                ans[cur++] = 'A' + i;
                cnt[i]--;
                if(dfs(cur, len)) return true;
            }
        }
    }
    return false;
}

int main() {
    //freopen("in.txt", "r", stdin);
    get_factor();
    while(scanf("%s%lld", &str, &tar) != EOF) {
        if(str[0] == '#' && tar == 0) break;

        memset(ans, 0, sizeof(ans));
        memset(cnt, 0, sizeof(cnt));

        int len = strlen(str);
        for(int i = 0; i < len; i++)
            cnt[str[i] - 'A']++;
        dfs(0, len);
        puts(ans);
    }
    return 0;
}