Java Program for Iterative Quick Sort

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// Java implementation of iterative quick sort
class IterativeQuickSort {
    void swap(int arr[], int i, int j)
    {
        int t = arr[i];
        arr[i] = arr[j];
        arr[j] = t;
    }
  
    /* This function is same in both iterative and
       recursive*/
    int partition(int arr[], int l, int h)
    {
        int x = arr[h];
        int i = (l - 1);
  
        for (int j = l; j <= h - 1; j++) {
            if (arr[j] <= x) {
                i++;
                // swap arr[i] and arr[j]
                swap(arr, i, j);
            }
        }
        // swap arr[i+1] and arr[h]
        swap(arr, i + 1, h);
        return (i + 1);
    }
  
    // Sorts arr[l..h] using iterative QuickSort
    void QuickSort(int arr[], int l, int h)
    {
        // create auxiliary stack
        int stack[] = new int[h - l + 1];
  
        // initialize top of stack
        int top = -1;
  
        // push initial values in the stack
        stack[++top] = l;
        stack[++top] = h;
  
        // keep popping elements until stack is not empty
        while (top >= 0) {
            // pop h and l
            h = stack[top--];
            l = stack[top--];
  
            // set pivot element at it's proper position
            int p = partition(arr, l, h);
  
            // If there are elements on left side of pivot,
            // then push left side to stack
            if (p - 1 > l) {
                stack[++top] = l;
                stack[++top] = p - 1;
            }
  
            // If there are elements on right side of pivot,
            // then push right side to stack
            if (p + 1 < h) {
                stack[++top] = p + 1;
                stack[++top] = h;
            }
        }
    }
  
    // A utility function to print contents of arr
    void printArr(int arr[], int n)
    {
        int i;
        for (i = 0; i < n; ++i)
            System.out.print(arr[i] + " ");
    }
  
    // Driver code to test above
    public static void main(String args[])
    {
        IterativeQuickSort ob = new IterativeQuickSort();
        int arr[] = { 4, 3, 5, 2, 1, 3, 2, 3 };
        ob.QuickSort(arr, 0, arr.length - 1);
        ob.printArr(arr, arr.length);
    }
}
/*This code is contributed by Rajat Mishra */

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Output:

1 2 2 3 3 3 4 5

The above mentioned optimizations for recursive quick sort can also be applied to iterative version.

1) Partition process is same in both recursive and iterative. The same techniques to choose optimal pivot can also be applied to iterative version.

2) To reduce the stack size, first push the indexes of smaller half.

3) Use insertion sort when the size reduces below a experimentally calculated threshold.

Please refer complete article on Iterative Quick Sort for more details!



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