recursive max function for list of list python

Question

I am trying to write a recursive function in order to find the max integer in a list of list. I know how to write a func for list of ints. Can anyone give me some tips for this?I I am thinking doing it with no Max function. ex. a = [1, [2,3], 4, [[2], 1]] find_max(a) ->4

Answer 1

I decided to tackle this with pure recursion, no loops. The following seems to do the trick for me:

def find_max(a_list):
    l = len(a_list)
    if l > 1:   # if there are multiple elements...
        l /= 2      # find the midpoint
        m1 = find_max(a_list[:l])   # find the max in the first half
        m2 = find_max(a_list[l:])   # find the max in the second half
        if m1 > m2:         # pick between them
            return m1
        else:
            return m2
    elif l < 1: # deal with empty lists
        return None
    else:       # we're down to one element...
        if isinstance(a_list[0], list):     # ...but it may be a list
            return find_max(a_list[0])      # if so, find its max
        else:
            return a_list[0]   # otherwise, a single element is trivially the max of its subset

Note that by splitting the subproblems in half rather than reducing by 1, this implementation should be robust against stack overflows even with large lists.

---

Now revised to deal with empty lists.

Answer 2

You can iterate through your list and call the MAX() function if data type is a list:

l = [[54,65,464,656,5],[568,49,7,8,4,3,3515],[312,64,598,46]]

def MAX(l):
    mx = None
    for item in l:
        if isinstance(item, list):
            tmp = MAX(item)
        else:
           tmp = item

       if mx < tmp:
          mx = tmp
    return mx