清北考前刷题day6早安

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>

#define N 1000010

using namespace std;
int n,a[N],cnt1,cnt2;
int read()
{
    int x=,f=;char c=getchar();
    while(c>''||c<''){if(c=='-')f=-;c=getchar();}
    while(c>=''&&c<=''){x=x*+c-'';c=getchar();}
    return ;
}
int main()
{
    scanf("%d",&n);
    for(int i=; i<=n; i++)a[i]=read();
    for(int i=; i<=n; i++)
    {
        if(a[i]==)cnt1++;
        if(a[i]==)
        {
            if(cnt1==)
            {
                puts("NO");return ;
            }
            else cnt2++,cnt1--;
        }
        if(a[i]==)
        {
            if(cnt1==||(cnt2==&&cnt1<))
            {
                puts("NO");
                return ;
            }
            if(cnt2) cnt2--,cnt1--;
            else cnt1-=;
        }
    }
    puts("YES");
    return ;
}

/*
贪心
反过来括号匹配。
*/
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cstdlib>
''
using namespace std;
const int maxn = ;
int p = ;
int z[maxn],a[maxn],n,m;

int main()
{
    scanf("%d",&n);
    for (int i = ; i<=n; i++)
        scanf("%d",&a[i]);
    scanf("%d",&m);
    int x;
    for (int i = ; i<=m; i++)
    {
        scanf("%d",&x);
        if (a[x]>) a[x] = -a[x];
    }
    for (int x = n; x>=; x--)
    {
        if (a[x]>)
        {
            if (z[p] == -a[x]) p--;
            else
            {
                a[x] = -a[x];
                p++;
                z[p]= a[x];
            }
        }
        else
        {
            p++;z[p]=a[x];
        }
    }
    if (p==)
    {
        for (int i = ; i<=n; i++)
        {
            if (a[i]>) printf("+%d ",a[i]);
            else printf("%d ",a[i]);
        }
    }
    else printf("NO\n");
    return ;
}

/*
Floyed
F[i][j][k]表示从i到j能否以k的方式到达。
K = 括号完全匹配  或者 缺少一个右括号
*/
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cstdlib>
using namespace std;
int n,m;
int g[][][],q[**][],t,x,y,z;
int main()
{
    freopen("maze.in","r",stdin);
    freopen("maze.out","w",stdout);
    scanf("%d%d",&n,&m);
    memset(g,,sizeof(g));
    t = ;
    for (int i = ; i<=m; i++)
    {
        scanf("%d%d%d",&x,&y,&z);
        if (z!=)
        {
            if (z<) z = abs(z);
            else z+=;
            g[x][y][z] = ;
            g[y][x][z] = ;
        }
        else
        {
            g[x][y][z] = ;
            g[y][x][z] = ;
            t++;
            q[t][] = x;
            q[t][] = y;
            q[t][] = ;
            t++;
            q[t][] = y;
            q[t][] = x;
            q[t][] = ;
        }
    }
    for (int i =; i<=n; i++)
    {
        g[i][i][] = ;
        t++;
        q[t][] = i;
        q[t][] = i;
        q[t][] = ;
    }
    for (int s = ; s<=t; s++)
    {
        int x = q[s][];
        int y = q[s][];
        int status = q[s][];
        if (status!=)
        {
            for (int i = ; i<=n; i++)
                if (g[i][x][status-] == )
                {
                    if (g[i][y][] ==)
                    {
                        g[i][y][] = ;
                        t++;
                        q[t][] = i;
                        q[t][] = y;
                        q[t][] = ;
                    }
                }
        }
        else
        {
            for (int i =; i<=n; i++)
            {
                if (g[i][x][] == && g[i][y][] == )
                {
                    g[i][y][] =;
                    t++;
                    q[t][]= i;
                    q[t][] = y;
                    q[t][]= ;
                }
                if (g[y][i][] ==  && g[x][i][] ==)
                {
                    g[x][i][] = ;
                    t++;
                    q[t][] = x;
                    q[t][] = i;
                    q[t][]= ;
                }
                for (int j = ; j<=; j ++)
                {
                    if (g[y][i][j] ==  && g[x][i][j+] == )
                    {
                        g[x][i][j+] = ;
                        t++;
                        q[t][] = x;
                        q[t][] = i;
                        q[t][] = j+;
                    }
                }
            }
        }
    }
    int tt = ;
    scanf("%d",&tt);
    for (; tt; tt--)
    {
        scanf("%d%d",&x,&y);
        if (g[x][y][] ==) printf("YES\n");
        else printf("NO\n");
    }
}