A combination algorithm in python
<pre><code>import copy

def combine(l, n):
    answers = []
    one = [0] * n
    def next_c(li = 0, ni = 0):
        if ni == n:
            answers.append(copy.copy(one))
            return
        for k in range(li, len(l)):
            one[ni] = l[k]
            next_c(k+1, ni+1)
    next_c()
    return answers

print(combine([1,2,3,4],2)) 
</code></pre>

<p>Recently I found this code on internet. It works very well. However, I do not know the details that how does it work？ So can anybody tell me how does it work? And how to understand a recursive code quickly? Thanks very much</p>