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quicksort-102-sp10

# quicksort-102-sp10 - Quicksort...

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Quicksort http://math.hws.edu/TMCM/java/xSortLab /

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Quicksort I To sort  a[left...right] : 1. if left < right: 1.1. Partition a[left...right] such that:     all a[left...p-1] are less than a[p], and         all a[p+1...right] are >= a[p] 1.2. Quicksort a[left...p-1] 1.3. Quicksort a[p+1...right] 2. Terminate Source: David Matuszek
Partitioning (Quicksort II) A key step in the Quicksort algorithm is partitioning the array We choose some (any) number  p  in the array to use as a pivot We partition the array into three parts: p numbers less than p numbers greater than or equal to p  p

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Partitioning II Choose an array value (say, the first) to use as the pivot Starting from the left end, find the first element that is greater than or equal to the pivot Searching backward from the right end, find the first element that is less than the pivot Interchange (swap) these two elements Repeat, searching from where we left off, until done
Partitioning To partition a[left...right]: 1. p = a[left];      l = left + 1;      r = right; 2.   while l < r, do 2.1.   while l < right && a[l] < p { l = l + 1; } 2.2.   while r > left && a[r] >= p { r = r – 1;} 2.3.   if l < r { swap a[l] and a[r] } 3.   a[left] = a[r];      a[r] = p; 4.  Terminate

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Example of partitioning choose pivot: 4 3 6 9 2 4 3 1 2 1 8 9 3 5 6 search: 4 3 6 9 2 4 3 1 2 1 8 9 3 5 6 swap: 4 3 3 9 2 4 3 1 2 1 8 9 6 5 6 search: 4 3 3 9 2 4 3 1 2 1 8 9 6 5 6 swap: 4 3 3 1 2 4 3 1 2 9 8 9 6 5 6 search:
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quicksort-102-sp10 - Quicksort...

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