Intel Code Challenge Final Round (Div. 1 + Div. 2, Combined) D. Dense Subsequence

传送门

吐槽：昨天诸事不宜，思路十分混乱，一道题也不会，真是太忧伤了。

分析：其实非常好想。枚举最后一个字母，然后暴力跑一遍。
如果跑到的字母比枚举的小，一定会放进去；如果跑了M个字母都没有比枚举的小，那么就找M个中和枚举的字母相等的放进去，更新一下状态；如果M个中都没有等于的，那么枚举的最后一个字母不成立。

代码：

/*****************************************************/
//#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <map>
#include <set>
#include <ctime>
#include <stack>
#include <queue>
#include <cmath>
#include <string>
#include <vector>
#include <cstdio>
#include <cctype>
#include <cstring>
#include <sstream>
#include <cstdlib>
#include <iostream>
#include <algorithm>
using namespace std;
#define   offcin        ios::sync_with_stdio(false)
#define   sigma_size    26
#define   lson          l,m,v<<1
#define   rson          m+1,r,v<<1|1
#define   slch          v<<1
#define   srch          v<<1|1
#define   sgetmid       int m = (l+r)>>1
#define   LL            long long
#define   ull           unsigned long long
#define   mem(x,v)      memset(x,v,sizeof(x))
#define   lowbit(x)     (x&-x)
#define   bits(a)       __builtin_popcount(a)
#define   mk            make_pair
#define   pb            push_back
#define   fi            first
#define   se            second

const int    INF    = 0x3f3f3f3f;
const LL     INFF   = 1e18;
const double pi     = acos(-1.0);
const double inf    = 1e18;
const double eps    = 1e-9;
const LL     mod    = 1e9+7;
const int    maxmat = 10;
const ull    BASE   = 31;

/*****************************************************/

const int maxn = 1e5 + 5;

char s[maxn];
std::vector<char> res;

bool check(char k, int l, int m) {
    res.clear(); int cnt = 0;
    stack<int> st;
    for (int i = 0; i < l; i ++) {
        if (s[i] == k) st.push(i);
        if (s[i] < k) {
            cnt = 0;
            res.pb(s[i]);
        }
        else cnt ++;
        if (cnt == m) {
            if (!st.empty() && i - st.top() < m) {
                res.pb(s[st.top()]);
                cnt = i - st.top();
                st.pop();
            }
            else return false;
        }
    }
    return true;
}

int main(int argc, char const *argv[]) {
    int m; cin>>m;
    scanf("%s", s);
    int len = strlen(s);
    for (char k = 'a'; k <= 'z'; k ++) if (check(k, len, m))
        break;
    sort(res.begin(), res.end());
    for (unsigned i = 0; i < res.size(); i ++) cout<<res[i];
    return 0;
}