LightOJ - 1060 nth Permutation(计数)

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题目大意：给你一个字符串，问这个字符串的字典序排第n的字符串是什么

解题思路：计数问题，依此判断每一位即可

#include <cstdio>#include <cstring>using namespace std;typedef long long LL;const int N = 30;LL A[N];int cnt[N];char str[N], ans[N];LL k;int cas = 1, len;void start() {    A[0] = A[1] = 1;    for (int i = 2; i < 21; i++)        A[i] = A[i - 1] * i;}LL Calculate() {    LL t1 = 0, t2 = 1;    for (int i = 0; i < 26; i++) {        t1 += cnt[i];        t2 *= A[cnt[i]];    }    return A[t1] / t2;}bool dfs(int pos, int cur, LL left) {    if (pos >= len) return true;    if (cnt[cur] == 0) return dfs(pos, cur + 1, left);    cnt[cur]--;    LL t = Calculate();    if (t >= left) {        ans[pos] = 'a' + cur;        return dfs(pos + 1, 0, left);    }    else {        cnt[cur]++;        return dfs(pos, cur + 1, left - t);    }}void solve() {    scanf("%s%lld", str, &k);    len = strlen(str);    memset(cnt, 0, sizeof(cnt));    for (int i = 0; i < len; i++)        cnt[str[i] - 'a']++;    if (Calculate() < k) printf("Case %d: Impossible\n", cas++);    else  {        ans[len] = '\0';        dfs(0, 0, k);        printf("Case %d: %s\n", cas++, ans);    }}int main() {    start();    int test;    scanf("%d", &test);    while (test--) solve();    return 0;}

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