【樹狀數組求第k小】POJ 2892

又一次用到樹狀數組求第k小，注意邊界情況

#define N 50005int c[N];int lowbit(int x){    return x&(-x);}void add(int x,int v){    while(x<N){        c[x]+=v;        x+=lowbit(x);    }}int sum(int x){    int ans = 0;    while(x){        ans+=c[x];        x-=lowbit(x);    }    return ans;}int find_k(int k){    int ans=0,cnt=0;    for(int i=log(double(N-1))/log(2.0);i>=0;i--){        ans += (1<<i);        if(ans>=N || cnt+c[ans]>=k){            ans-=(1<<i);        } else cnt+=c[ans];    }    return ans+1;}int main(){    int n,m;    while(scanf("%d%d",&n,&m)!=-1){        int i,j;        memset(c,0,sizeof(c));        stack<int> st;        add(1,1);//注意邊界，都往後移一位        add(n+2,1);//同上        while(m--){            char str[2];            scanf("%s",str);            int pos;            if(str[0]=='D'){                scanf("%d",&pos);                pos++;                add(pos,1);                st.push(pos);            } else if(str[0]=='R'){                pos = st.top();st.pop();                add(pos,-1);            } else {                scanf("%d",&pos);                pos++;                int k = sum(pos);                int a = find_k(k);                int b = find_k(k+1);                if(k-sum(pos-1)==1){                    printf("0\n");                } else printf("%d\n",b-a-1);            }        }    }    return 0;}