algos/quick_select/QuickSelect.java at master · OpenSourceCodeater/algos · GitHub

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publicclassQuickSelect {
/*
 * partition function
 * array : array on which partitioning has to be done
 * left : left index of the partitioning subarray
 * right : right index of the partitioning subarray
 * pivotIndex : pivot index from which partition has to done
 * return : the index of the last element of the left subarray
 */
privatestaticintpartition(int[] array, intleft, intright, intpivotIndex) {
intpivotValue = array[pivotIndex];
inttemp = array[right];
array[right] = array[pivotIndex];
array[pivotIndex] = temp;
intstoreIndex = left;
while (left < right) {
if (array[left] < pivotValue) {
temp = array[storeIndex];
array[storeIndex] = array[left];
array[left] = temp;
storeIndex++;
 }
left++;
 }
temp = array[right];
array[right] = array[storeIndex];
array[storeIndex] = temp;
returnstoreIndex;
 }

/*
 * Quick Select function
 * left : left index of the subarray
 * right : right index of the subarray
 * pos : position to find the element using quick sort
 * return : the value of element at pos place in the sorted array
 */
publicstaticintquickSelect(int[] array, intleft, intright, intpos) {
intpivotIndex;
if(pos < 0 || pos >= array.length) {
thrownewIndexOutOfBoundsException("index: " + pos);
 }
if (left == right) {
returnarray[left];
 }
pivotIndex = right - 1;
pivotIndex = partition(array, left, right, pivotIndex);
if (pos == pivotIndex) {
returnarray[pivotIndex];
 }
elseif (pos < pivotIndex) {
returnquickSelect(array, left, pivotIndex - 1, pos);
 }
else {
returnquickSelect(array, pivotIndex + 1, right, pos);
 }
 }

publicstaticvoidmain(String[] args) {
int[] array = {10, 5, 1, 6, 7, 3, 2, 4, 8, 9};
System.out.println(quickSelect(array, 0, array.length - 1, 3));
 }

}