Given an array [a1b7c3d2]
convert to [abcd1732]
with O(1)
space and O(n)
time i.e. put the letters on the left and digits on the right such that their relative order is the same. I can think of an O(nlogn)
algorithm, but not better. Can somebody please help?
AFAIK it can't be done. This is essentially a single step of the RADIX sort algorithm. And AFAIK stable RADIX sort can't be done in-place.
edit Wikipedia agrees with me (for what that's worth):
http://en.wikipedia.org/wiki/Radix_sort#Stable_MSD_radix_sort_implementations
MSD Radix Sort can be implemented as a stable algorithm, but requires the use of a memory buffer of the same size as the input array
Edit2
If the input is always in pairs of letter-number, then the solution is quite simple, as we always know which character should go where:
for i=0...n/2-1
tmp=array[i]
if tmp is a letter
continue // nothing to do, we have a letter already!
index=i
do
// we have the wrong think at index! Where is it supposed to be?
if (index is even) // the wrong thing is a letter
index=index/2
else // the wrong thing is a number
index=n/2+index/2
// we found where temp should go! Lets put it there!
// But we keep what was already there and look for its place next iteration
tmp2=array[index]
array[index]=tmp
tmp=tmp2
while index!=i
It might look quadratic, as for each i
we do the while
, but actually every element is only moved once hence it's linear.
First off, this is really a duplicate of this SO question, just stated a bit differently.
(Since your problem could really be considered a stable 0-1 sort)
Unfortunately I can't figure out the algorithm, nor find any simple pseudocode, but if you'd like the algorithm is descibed here: http://www.diku.dk/~jyrki/Paper/KP1992bJ.pdf
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