$O(N)$

按照小写字母、数字、大写字母的顺序将三类字符从前向后输出。

#include<bits/stdc++.h>

using namespace std;

typedef long long LL;
typedef pair<LL, LL> PII;

#define x first
#define y second

const int N = 2e5 + 10, M = 30;

int a[N];

void solve() 
{
    int n;
    string s;
    cin >> n >> s;
    for(int i = 0; i < n; i ++)
        if(s[i] >= 'a' && s[i] <= 'z')
            cout << s[i];
    for(int i = 0; i < n; i ++)
        if(s[i] >= '0' && s[i] <= '9')
            cout << s[i];
    for(int i = 0; i < n; i ++)
        if(s[i] >= 'A' && s[i] <= 'Z')
            cout << s[i];
}            

int main()
{
    int t = 1;
    // cin >> t;
    while(t --)
        solve();
}