Mix two lists python

You could implement a greedy algorithm which tries to yield the most common item as frequently as possible (i.e. up to twice), and then yield the next most common item when necessary.

This has two advantages over search by random shuffle:

• The greedy algorithm is much faster as the length of items increases:

  In [223]: %timeit list(intermix([1]*10+[2]*5))
  10000 loops, best of 3: 39.8 µs per loop
  
  In [222]: %timeit intermix_random([1]*10+[2]*5)
  100 loops, best of 3: 6.85 ms per loop
  

• It can identify when there is no solution, whereas a random shuffle search loops forever if visited shuffles are not cached.

---

import collections
def intermix(items, nconsecutive=2):
    counter = collections.Counter(items)
    # sort from most common to least common
    items = sorted(items, key=counter.get, reverse=True)
    N = len(items)
    count = 0
    # remember the last two values
    last = []
    for i in range(N):
        val = items[i]
        if len(last) < nconsecutive:
            if last and val == last[-1]:
                last.append(val)
            else:
                last = [val]
            counter[val] -= 1
            yield val
        else:
            # last is full; find a different value
            for j in range(i, N):
                if items[j] != last[-1]:
                    items[i], items[j] = items[j], items[i]
                    val = items[i]
                    last = [val]
                    counter[val] -= 1
                    # as items are yielded, the meaning of "most common" can change.
                    items[i+1:] = sorted(items[i+1:], key=counter.get, reverse=True)
                    yield val
                    break
            else:
                raise ValueError('No solution possible')

---

In [184]: list(intermix([1,1,1,1,1,2,2,2,2,2]))
Out[184]: [1, 1, 2, 2, 1, 2, 2, 1, 2, 1]

In [185]: list(intermix([1,0,1,1,2,1,0,1,1,1,2]))
Out[185]: [1, 1, 0, 1, 1, 2, 1, 1, 2, 1, 0]

In [186]: list(intermix([1,0,1,1,2,1,0,1,1,1,1,1,1,2]))
Out[186]: [1, 1, 0, 1, 1, 2, 1, 1, 2, 1, 1, 0, 1, 1]

In [187]: list(intermix([1,0,1,1,2,1,0,1,1,1,1,1,1,1,2]))
ValueError: No solution possible

In [188]: list(intermix([1,0,1,1,2,1,0,1,1,1,1,1,1,1,2], nconsecutive=3))
Out[188]: [1, 1, 1, 0, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 0]