【Codeforces 724 D Dense Subsequence】+ 贪心

D. Dense Subsequence
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output

You are given a string s, consisting of lowercase English letters, and the integer m.

One should choose some symbols from the given string so that any contiguous subsegment of length m has at least one selected symbol. Note that here we choose positions of symbols, not the symbols themselves.

Then one uses the chosen symbols to form a new string. All symbols from the chosen position should be used, but we are allowed to rearrange them in any order.

Formally, we choose a subsequence of indices 1 ≤ i1 < i2 < … < it ≤ |s|. The selected sequence must meet the following condition: for every j such that 1 ≤ j ≤ |s| - m + 1, there must be at least one selected index that belongs to the segment [j,  j + m - 1], i.e. there should exist a k from 1 to t, such that j ≤ ik ≤ j + m - 1.

Then we take any permutation p of the selected indices and form a new string sip1sip2… sipt.

Find the lexicographically smallest string, that can be obtained using this procedure.
Input

The first line of the input contains a single integer m (1 ≤ m ≤ 100 000).

The second line contains the string s consisting of lowercase English letters. It is guaranteed that this string is non-empty and its length doesn’t exceed 100 000. It is also guaranteed that the number m doesn’t exceed the length of the string s.
Output

Print the single line containing the lexicographically smallest string, that can be obtained using the procedure described above.
Examples
Input

3
cbabc

Output

a

Input

2
abcab

Output

aab

Input

3
bcabcbaccba

Output

aaabb

Note

In the first sample, one can choose the subsequence {3} and form a string “a”.

In the second sample, one can choose the subsequence {1, 2, 4} (symbols on this positions are ‘a’, ‘b’ and ‘a’) and rearrange the chosen symbols to form a string “aab”.

贪心，在[l + 1, l +ｍ－１]里选出最小，字典序最下，把比选中的最大的字符小的都加上？

AC代码：

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;const int K = 1e5 + 10;bool vis[K];char s[K],st[K],ss ='a';int main(){    int N;    scanf("%d %s",&N,s);    int nl = strlen(s),pl = 0,kl = 0;    while(pl + N - 1 < nl){        int ml = pl;        for(int i = pl; i <= pl + N - 1; i++)            if(s[i] <= s[ml]) ml = i;        st[++kl] = s[ml],ss = max(ss,s[ml]),pl = ml + 1,vis[ml] = 1;    }    for(int i = 0 ; i < nl ; i++)        if(!vis[i] && s[i] < ss) st[++kl] = s[i];    sort(st + 1, st + 1 + kl);    printf("%s\n",st + 1);    return 0;}