poj 2892 树状数组 二分

代码很好懂，不解释了

#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;
}