QUICKSORT: METHOD

1
QUICKSORT METHOD

• public static void quickSort (int left, int
  right, int arr)
• // use middle value of current partition to
  split it
• int i left, j right, pVal arr (left
  right) / 2
• do
• // move index vals towards middle while values
  conform to partition
• while (arr i lt pVal) i
• while (arr j gt pVal) j--
• // swap "out of partition" elements
• if (i lt j)
• if (i lt j) swap (i, j, arr)
• i
• j--
• // end if
• while (i lt j)
• // invoke quicksort on resulting partitions
• if (left lt j) quickSort (left, j, arr)
• if (i lt right) quickSort (i, right, arr)
• // end quickSort method

2
QUICKSORT ILLUSTRATIONS

• next 2 pages show quicksort method in operation
• first, listing array contents as they change
• underlining marks chosen partitioning value 21
• bolding marks values selected for swap 24 18
• bold vertical borders mark partitions of array
• second, graphical representation of recursive
  calls
• red green arrows mark recursion on left right
  partitions
• dashed blue arrows mark base case, recursion
  ending

3
QUICKSORT TRACE

4
QUICKSORT CALL TRACE

• Quick Sort execution tree (for the above example)
• RED recurse on left partition
• GREEN recurse on right partition
• BLUE recursion ends
• test for (left lt j) fails
• or test for (i lt right) fails