Check whether given preorder is BST or not

def isBSTPreorder(arr):
    i=0
    N=len(arr)
    'bound algorithm'
    def create(l,r):
        nonlocal i
        if i==N:return
        if l<=arr[i]<=r:
            t=arr[i]
            i+=1
            create(l,t-1)
            create(t+1,r)
        else:return
    create(float('-inf'),float('inf'))
    return 1 if i==N else 0
'coding ninjas'