PAT1043 Is It a Binary Search Tree 水题

#include<algorithm>
#include<cstdio>
#include<iostream>
#include<vector>
#include<queue>
#include<map>
#include<string>
#include<cstring>
#include<stack>
#define N 100005
#define ll long long
#define pb push_back
#define eps 1e-9
using namespace std;
int cnt,ok;
int a[N];
struct node
{
	int data,ls,rs;
}tree[N];
int build(int l,int r,int s)
{
	if(l>r)return -1;
	int i = l+1,j;
	if(!s)
	{
		while(a[i]<a[l]&&i<=r)i++;
		for(j = i;j<=r;j++)
			if(a[j]<a[l])
			{
				ok = 0;
				return 0;
			}
	}
	else 
	{
		while(a[i]>=a[l]&&i<=r)i++;
		for(j = i;j<=r;j++)
			if(a[j]>=a[l])
			{
				ok = 0;
				return 0;
			}
	}
	int id = ++cnt;
	tree[id].data = a[l];
	tree[id].ls = build(l+1,i-1,s);
	tree[id].rs = build(i,r,s);
	return id;
}
void dfs(int rt)
{
	if(rt == -1)return;
	dfs(tree[rt].ls);
	dfs(tree[rt].rs);
	if(rt == 1)printf("%d\n",tree[rt].data);
	else printf("%d ",tree[rt].data);
}
int main()
{
	int n,i,j;
	while(scanf("%d",&n)!=EOF)
	{
		cnt = 0;
		ok = 1;
		for(i = 1;i<=n;i++)scanf("%d",&a[i]);
		build(1,n,0);
		if(!ok)
		{
			ok = 1;
			build(1,n,1);
		}
		if(!ok)printf("NO\n");
		else 
		{
			printf("YES\n");
			dfs(1);
		}
	}
	return 0;
}