All possible ways to interleave two strings

Several other solutions have already been posted, but most of them generate the full list of interleaved strings (or something equivalent to it) in memory, making their memory usage grow exponentially as a function of the input length. Surely there must be a better way.

Enumerating all ways to interleave two sequences, of length a and b respectively, is basically the same as enumerating all a+b bit integers with exably b bits set. Each such integer corresponds to a distinct way to interleave the sequences, obtained by replacing every 0 bit with an element of the first sequence, and every 1 bit with an element of the second sequence.

Conveniently, there's a clever and efficient way to calculate the next integer with the same number of bits set, which we can use to generate all such integers. So let's do that first:

def bit_patterns(m, n):
    """Generate all m-bit numbers with exactly n bits set, in ascending order.
    See http://www.geeksforgeeks.org/next-higher-number-with-same-number-of-set-bits/
    """
    patt = (1 << int(n)) - 1
    if patt == 0: yield 0; return  # loop below assumes patt has at least one bit set!
    while (patt >> m) == 0:
        yield patt
        lowb = patt & -patt  # extract the lowest bit of the pattern
        incr = patt + lowb   # increment the lowest bit
        diff = patt ^ incr   # extract the bits flipped by the increment
        patt = incr + ((diff // lowb) >> 2)  # restore bit count after increment

Now we can use this generator to generate all ways to interleave any two sequences:

def interleave(a, b):
    """Generate all possible ways to interleave two sequences."""
    m = len(a) + len(b)
    n = len(a)
    for pattern in bit_patterns(m, n):
        seq = []
        i = j = 0
        for k in range(m):
            bit = pattern & 1
            pattern >>= 1
            seq.append(a[i] if bit else b[j])
            i += bit
            j += 1-bit
        yield seq

Note that, in order to try to be as generic as possible, this code takes arbitrary sequence types and returns lists. Strings are sequences in Python, so you can pass them in just fine; to convert the generated lists back into strings, you can concatenate their elements e.g. with "".join(), like this:

foo = "ABCD"
bar = "1234"
for seq in interleave(foo, bar):
    print("".join(seq))

There we go: a fully non-recursive efficient generator-based solution that uses very little memory even for long inputs, and only generates each output once (thus requiring no inefficient duplicate elimination step). And it even works in both Python 2 and 3.

The Idea

Let the two strings you want to interleave be s and t. We will use recursion to generate all the possible ways to interleave these two strings.

If at any point of time we have interleaved the first i characters of s and the first j characters of t to create some string res, then we have two ways to interleave them for the next step-

1. Append the i+1 th character of s to res
2. Append the j+1 th character of t to res

We continue this recursion till all characters of both the strings have been used and then we store this result in a list of strings lis as in the code below.

The Code

def interleave(s, t, res, i, j, lis):
    if i == len(s) and j == len(t):
        lis.append(res)
        return
    if i < len(s):
        interleave(s, t, res + s[i], i + 1, j, lis)
    if j < len(t):
        interleave(s, t, res + t[j], i, j + 1, lis)

l = []
s = "ab"
t = "cd"
interleave(s, t, "", 0, 0, l)
print l

Output

['abcd', 'acbd', 'acdb', 'cabd', 'cadb', 'cdab']

This implementation is as efficient as we can get (at least asymptotically) since we never generate the same string twice.