Find left immediate greater and right immediate greater for every element in array

from collections import deque
def find(a):
    q=deque()
    n=len(a)
    pre,pos=[-1]*n,[n]*n
    for i,val in enumerate(a):
        while q and a[q[-1]]<=val:#top<=cur decreasing stack
            idx=q.pop()
            pos[idx]=i
        if q:
            pre[i]=q[-1]
        q.append(i)#This I forget always Keep remember this
    print(pre,pos)
a=[10, 4, 2, 20, 40, 12, 30]
find(a)