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Thread: recursive question
 10122013, 07:10 AM #1Member
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recursive question
Hi,
This is part of the QuickSort algorithmn I am trying to understand.
public static void sort(double[] a, int begin, int end){
if((end  begin) >= 1)
{
int splitPoint = split(a, begin, end);
sort(a, begin, splitPoint);
sort(a, splitPoint + 1, end);
}
}
How does the recursive call work in this case ? After getting a return value from split()
Does it execute sort(a, begin, splitPoint) until base case is met, then back track each stack ? then go to sort(a, splitPoint + 1, end) ?
Or does it do sort(a, begin, splitPoint) follow by sort(a, splitPoint + 1, end) until base case is met ?
Thanks
 10122013, 08:40 AM #2
 10122013, 09:32 AM #3
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Re: recursive question
A proof 'by intimidation' runs as follows: suppose the sort( ... ) method works; the real 'intelligence' is in the splitPoint( ... ) method, i.e. it has to find an index n such that for all j < n, array[j] <= array[n] and for all j > n, array[j] >= array[n] (iow, elements to the left of n have to be smaller than array[n] and elements to the right of n have to be larger than array[n]). If you sort the elements to the left of n and the elements to the right of n you have sorted the entire array.
An example: array= { 3, 2, 1, 4, 7, 6, 5 } and you pick 4 as the pivot; the elements to the left of 4 are smaller than 4 and all elements to the right of 4 are larger than 4; if you can sort the elements { 3, 2, 1 } and { 7, 6, 5 } you have sorted the entire array.
kind regards,
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