bupt 1849 Counting Sequences

#include  < stdio.h >
#include  < algorithm >
#include  < string .h >
using   namespace  std;
struct  point {
    int x,y;
} p[ 100010 ];
int  c[ 100010 ],n,d,a[ 100010 ],map[ 100010 ],t;
bool  cmp(point p0,point p1) {
    return p0.x<p1.x;
}
int  lowbit( int  x) {
    return x&(-x);
}
int  sum( int  x) {
    int s=0;
    while(x){
        s=(s+c[x])%9901;
        x-=lowbit(x);
    }
    return s;
}
int  update( int  x, int  y) {
    while(x<=n){
        c[x]=(c[x]+y)%9901;
        x+=lowbit(x);
    }
}
int  find( int  x, bool  y) {
    int l=1,r=n,m,ans=-1;
    if(y){
        while(l<=r){
            m=(l+r)>>1;
            if(p[m].x>=x){
                ans=m;
                r=m-1;
            }
            else l=m+1;
        }
    }else{
        while(l<=r){
            m=(l+r)>>1;
            if(p[m].x<=x){
                ans=m;
                l=m+1;
            }
            else r=m-1;
        }
    }
    return ans;
}
int  main() {
    while(scanf("%d%d",&n,&d)!=EOF){
        memset(c,0,sizeof(c[0])*(n+1));
        for(int i=1;i<=n;i++){
            scanf("%d",&a[i]);
            p[i].x=a[i];
            p[i].y=i;
        }
        sort(p+1,p+n+1,cmp);
        for(int i=1;i<=n;i++)map[p[i].y]=i;
        for(int i=1;i<=n;i++){
            int l=find(a[i]-d,1),r=find(a[i]+d,0);
            if(l+1&&r+1&&l<=r){
                t=(9901+sum(r)-sum(l-1)+1)%9901;//+1巧妙之处，用于标记是否被更新，答案再减去n即可 
                update(map[i],t);
            }
        }
        printf("%d\n",(9901*20+sum(n)-n)%9901);
    }
    return 0;
}