def partition(arr,left,right):
    i=left
    j=right
    pivot = arr[(left+right)//2]
    while i<=j:
        while arr[i] < pivot:
            i+=1
        while arr[j] > pivot:
            j-=1
        if i<=j:
            arr[i],arr[j] = arr[j],arr[i]
            i+=1
            j-=1
    return i
partition([5,4,3,2,1],0,4)