poj 2892 树状数组 二分

代码很好懂，不解释了

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#include<cstdio>

#include<cstring>

const int maxn = 50010;

int c[maxn];

void add(int x,int d)

{

    for(;x<maxn;x+=x&-x)

        c[x]+=d;

}

int sum(int x)

{

    int ans=0;

    for(;x>0;x-=x&-x)

        ans+=c[x];

    return ans;

}

int bin(int val,int n)

{

    int l=1,r=n+1,best=n+2;

    while(l<=r)

    {

        int m=(l+r)>>1;

        if(sum(m)>=val)

        {

            best=m;

            r=m-1;

        }

        else l=m+1;

    }

    return best;

}

int st[maxn];

bool flag[maxn];



int main()

{

    int n,m,i,j,x;

    char op[5];

    while(scanf("%d%d",&n,&m)!=EOF)

    {

        int tot=0;

        memset(flag,0,sizeof(flag));

        memset(c,0,sizeof(c));

        while(m--)

        {

            scanf("%s",op);

            if(op[0]=='D')

            {

                scanf("%d",&x);

                x++;

                st[++tot]=x;

                flag[x]=true;

                add(x,1);

            }

            else if(op[0]=='R')

            {

                 x=st[tot--];

                 flag[x]=false;

                 add(x,-1);

            }

            else 

            {

                scanf("%d",&x);

                x++;

                if(flag[x]){ printf("0\n");continue;}

                x=sum(x);

                int pos1=bin(x,n);

                int pos2=bin(x+1,n);

                printf("%d\n",pos2-pos1-1);

            }

        }

    }

    return 0;

}