RT，经测试：

#include <map>
#include <stack>
#include <queue>
#include <cmath>
#include <ctime>
#include <cstdio>
#include <vector>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#define D double
#define LD long double
#define LL long long
#define ULL unsigned long long
#define S string
#define fi first
#define se second
#define mp make_pair
using namespace std;
S s, tmp;
S ans;

int a[10001] = {0};//这

int v[10001] = {0};//这

int main()
{
	int n, p;
	cin >> n;
	cin >> s;
	tmp = s;
	cin >> p;
	for (int i = 0; i < n; i++)
	{
		a[i] = s[i] - 'a';
	}
	sort(a, a + n);
	for (int i = 0; i < n; i++)
	{
		s[i] = a[i] + 'a';
	}
	int sta = 0;
	for (int i = 0; i < n; i++)
	{
		if (s[i] == tmp[p - 1])
		{
			sta = i;
			v[i] = 1;
			break;
		}
	}
	for (int i = n - 1; i >= 0; i--)
	{
		ans[i] = tmp[sta];
		for (int j = n - 1; j >= 0; j--)
		{
			if (tmp[sta] == s[j] && v[j] != 1)
			{
				sta = j;
				v[j] = 1;
				break;
			}
		}
	}
	for (int i = 0; i < n; i++)
	{
		cout << ans[i];
	}
	return 0;
}

能A

#include <map>
#include <stack>
#include <queue>
#include <cmath>
#include <ctime>
#include <cstdio>
#include <vector>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#define D double
#define LD long double
#define LL long long
#define ULL unsigned long long
#define S string
#define fi first
#define se second
#define mp make_pair
using namespace std;
S s, tmp;
S ans;

int v[10001] = {0};//这

int a[10001] = {0};//这

int main()
{
	int n, p;
	cin >> n;
	cin >> s;
	tmp = s;
	cin >> p;
	for (int i = 0; i < n; i++)
	{
		a[i] = s[i] - 'a';
	}
	sort(a, a + n);
	for (int i = 0; i < n; i++)
	{
		s[i] = a[i] + 'a';
	}
	int sta = 0;
	for (int i = 0; i < n; i++)
	{
		if (s[i] == tmp[p - 1])
		{
			sta = i;
			v[i] = 1;
			break;
		}
	}
	for (int i = n - 1; i >= 0; i--)
	{
		ans[i] = tmp[sta];
		for (int j = n - 1; j >= 0; j--)
		{
			if (tmp[sta] == s[j] && v[j] != 1)
			{
				sta = j;
				v[j] = 1;
				break;
			}
		}
	}
	for (int i = 0; i < n; i++)
	{
		cout << ans[i];
	}
	return 0;
}

只能过前两个点，求大佬解答为啥调换一下顺序就会错啊