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=0,f=1;char c=getchar();
    while(c>'9'||c<'0'){if(c=='-')f=-1;c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}
    return 0;
}
int main()
{
    scanf("%d",&n);
    for(int i=1; i<=n; i++)a[i]=read();
    for(int i=1; i<=n; i++)
    {
        if(a[i]==5)cnt1++;
        if(a[i]==10)
        {
            if(cnt1==0)
            {
                puts("NO");return 0;
            }
            else cnt2++,cnt1--;
        }
        if(a[i]==20)
        {
            if(cnt1==0||(cnt2==0&&cnt1<3))
            {
                puts("NO");
                return 0;
            }
            if(cnt2) cnt2--,cnt1--;
            else cnt1-=3;
        }
    }
    puts("YES");
    return 0;
}

 

 

/*
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 = 1000007;
int p = 0;
int z[maxn],a[maxn],n,m;

int main()
{
    scanf("%d",&n);
    for (int i = 1; i<=n; i++)
        scanf("%d",&a[i]);
    scanf("%d",&m);
    int x;
    for (int i = 1; i<=m; i++)
    {
        scanf("%d",&x);
        if (a[x]>0) a[x] = -a[x];
    }
    for (int x = n; x>=1; x--)
    {
        if (a[x]>0)
        {
            if (z[p] == -a[x]) p--;
            else
            {
                a[x] = -a[x];
                p++;
                z[p]= a[x];
            }
        }
        else
        {
            p++;z[p]=a[x];
        }
    }
    if (p==0)
    {
        for (int i = 1; i<=n; i++)
        {
            if (a[i]>0) printf("+%d ",a[i]);
            else printf("%d ",a[i]);
        }
    }
    else printf("NO\n");
    return 0;
}

 

 

 

/*
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[105][105][40],q[105*105*40][3],t,x,y,z;
int main()
{
    freopen("maze.in","r",stdin);
    freopen("maze.out","w",stdout);
    scanf("%d%d",&n,&m);
    memset(g,0,sizeof(g));
    t = 0;
    for (int i = 1; i<=m; i++)
    {
        scanf("%d%d%d",&x,&y,&z);
        if (z!=0)
        {
            if (z<0) z = abs(z);
            else z+=10;
            g[x][y][z] = 1;
            g[y][x][z] = 1;
        }
        else
        {
            g[x][y][z] = 1;
            g[y][x][z] = 1;
            t++;
            q[t][0] = x;
            q[t][1] = y;
            q[t][2] = 0;
            t++;
            q[t][0] = y;
            q[t][1] = x;
            q[t][2] = 0;
        }
    }
    for (int i =1; i<=n; i++)
    {
        g[i][i][0] = 1;
        t++;
        q[t][0] = i;
        q[t][1] = i;
        q[t][2] = 0;
    }
    for (int s = 1; s<=t; s++)
    {
        int x = q[s][0];
        int y = q[s][1];
        int status = q[s][2];
        if (status!=0)
        {
            for (int i = 1; i<=n; i++)
                if (g[i][x][status-10] == 1)
                {
                    if (g[i][y][0] ==0)
                    {
                        g[i][y][0] = 1;
                        t++;
                        q[t][0] = i;
                        q[t][1] = y;
                        q[t][2] = 0;
                    }
                }
        }
        else
        {
            for (int i =1; i<=n; i++)
            {
                if (g[i][x][0] ==1 && g[i][y][0] == 0)
                {
                    g[i][y][0] =1;
                    t++;
                    q[t][0]= i;
                    q[t][1] = y;
                    q[t][2]= 0;
                }
                if (g[y][i][0] == 1 && g[x][i][0] ==0)
                {
                    g[x][i][0] = 1;
                    t++;
                    q[t][0] = x;
                    q[t][1] = i;
                    q[t][2]= 0;
                }
                for (int j = 1; j<=10; j ++)
                {
                    if (g[y][i][j] == 1 && g[x][i][j+20] == 0)
                    {
                        g[x][i][j+20] = 1;
                        t++;
                        q[t][0] = x;
                        q[t][1] = i;
                        q[t][2] = j+20;
                    }
                }
            }
        }
    }
    int tt = 0;
    scanf("%d",&tt);
    for (; tt; tt--)
    {
        scanf("%d%d",&x,&y);
        if (g[x][y][0] ==1) printf("YES\n");
        else printf("NO\n");
    }
}