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    <p>You can figure out how <code>printList</code> behaves by drawing a recursion tree. Each node consists of two elements: an <code>alist</code> and a <code>blist</code>. The root has the <code>alist</code> with the initial sequence of items you want to permute, and an empty <code>blist</code>. Each node of the tree has one branch for each element of that node's <code>alist</code>; you move from a 'father' node to each one of its 'children' by choosing an element from the father's <code>alist</code> and:</p> <ul> <li>assigning to the child's <code>alist</code> the father's <code>alist</code> <em>minus that element</em>;</li> <li>assigning to the child's <code>blist</code> the father's <code>blist</code> plus that element <em>appended to its end</em>.</li> </ul> <p>The leafs have an empty <code>alist</code>, and since following different paths from the root to the leafs you have to choose elements from the root's <code>alist</code> in different orders, the <code>blist</code> of the leafs themselves contain all the various permutations of the root's <code>alist</code>.</p> <p>For example (<code>[abc],[] == alist,blist</code>):</p> <pre><code> [abc],[] / | \ a/ b| \c / | \ [bc],[a] [ac],[b] [ab],[c] / \ b/ \c / \ [c],[ab] [b],[ac] | | c| |b | | [],[abc] [],[acb] def printList(alist, blist=[]): # if alist is empty, we are in a 'leaf' in the recursion tree; # then blist contains one permutation; print it if not len(alist): print ''.join(blist) # ELSE, for each possible position in alist, for i in range(len(alist)): # move the element at that position from alist to the end of blist blist.append(alist.pop(i)) # go to the 'children' node and do the printing job for its subtree printList(alist, blist) # then move back the element from the end of blist to its original # position in alist, so we can continue with the for loop # without altering alist alist.insert(i, blist.pop()) </code></pre>
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    1. USFederico A. Ramponi
    1. USFederico A. Ramponi
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    1. CO+1 for the ascii tree (and the good explanation of course ;-)
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      1. USChristopheD
    2. COIts pretty clear when u put it like that, thanks for helping a newbie out! Vote up when i hit 15!
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      1. USKev
 

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