Educational Codeforces Round 64 (Rated for Div. 2) (线段树二分)

#include<bits/stdc++.h>
#define maxn 200005
#define mod 1000000007
using namespace std;
typedef long long ll;
struct sss
{
    int l,r;
    int val;
}tree[maxn*4];
int n;
int a[maxn];
int pos[maxn];
void build(int cnt,int l,int r)
{
    if(l==r){
        tree[cnt].l=l;
        tree[cnt].r=r;
        tree[cnt].val=a[l];
        return;
    }
    tree[cnt].l=l;
    tree[cnt].r=r;
    int mid=(l+r)/2; 
    build(cnt*2,l,mid);
    build(cnt*2+1,mid+1,r);
    tree[cnt].val=max(tree[cnt*2].val,tree[cnt*2+1].val); 
} 
int query(int cnt,int l,int r)
{
    if(l<=tree[cnt].l&&r>=tree[cnt].r) return tree[cnt].val;
    int mid=(tree[cnt].l+tree[cnt].r)/2;
    if(r<=mid)  return query(cnt*2,l,r);
    else if(l>mid) return query(cnt*2+1,l,r);
    else{
        return max(query(cnt*2,l,mid),query(cnt*2+1,mid+1,r));
    } 
    
}
int dl(int x)
{
    int val=a[x];
    int l=1;
    int r=x;
    while(r-l>1){
        int mid=(l+r)/2;
        int max_val=query(1,mid,x); //如果满足这个区间的最大值是这个数就继续扩张
        if(max_val==val)
        {
            r=mid;
        }
        else{
            l=mid;
        }
    }
    int max_val=query(1,l,x);
    if(max_val==val) return l;
    else return r; 
}
int dr(int x)
{
    int val=a[x];
    int l=x;
    int r=n;
    while(r-l>1){
        int mid=(l+r)/2;
        int max_val=query(1,x,mid); 
        if(max_val==val)
        {
            l=mid;
        }
        else{
            r=mid;
        }
    }
    int max_val=query(1,x,r); 
    if(max_val==val) return r;
    else return l;
}
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
    {
       scanf("%d",&a[i]); 
       pos[a[i]]=i;
    }
    build(1,1,n);
    int ans=0;
    for(int i=1;i<=n;i++){
        int l=dl(i);//找出左边界
        int r=dr(i);//找出右边界
        if(i-l<r-i){//枚举短的那一边
            for(int j=l;j<i;j++){
                int other=a[i]-a[j];
                if (other<=0||other>n)continue;
                ans+=pos[other]>i&&pos[other]<=r;//确定令一边是否在这个区间里面
            }
        }
        else{
            for(int j=i+1;j<=r;j++){
                int other=a[i]-a[j];
                if (other<=0||other>n)continue;
                ans+=pos[other]<i&&pos[other]>=l;
            }
        }
    } 
    printf("%d",ans);
}