UVa 941 - Permutations

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題目：已知一個字母的有序集合，求出排第n的串。

分析：組合數學、字符串。康拓展開。

說明：注意對字符排序，UVa終於進前300名了╮(╯▽╰)╭。

#include <algorithm>#include <iostream>#include <cstdlib>#include <cstring>#include <cstdio>using namespace std;char buf[22];long long factorial[22];void cantor_expansion(long long n, int m){for (int i = m-1; i >= 0; -- i) {long long index = n/factorial[i], count = 0;for (int j = 0; j < m; ++ j) {if (buf[j]) {if (count == index) {printf("%c",buf[j]);buf[j] = 0;break;}else {count ++;}}}n %= factorial[i];}puts("");}int main(){factorial[0] = 1LL;for (int i = 1; i < 21; ++ i)factorial[i] = factorial[i-1]*i;long long n, m;while (~scanf("%lld",&m))while (m --) {scanf("%s%lld",buf,&n);sort(buf, buf+strlen(buf));cantor_expansion(n, strlen(buf));}    return 0;}

測試數據：

1abcdefghijklmnopqrst888888888888888881abcdefghijklmnopqrst2432902008176639999

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