69 lines
2 KiB
Java
69 lines
2 KiB
Java
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package zutil.algo;
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import zutil.algo.sort.sortable.SortableDataList;
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/**
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* This algorithm is a modified QuickSort
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* to find the k smallest or biggest value
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* http://en.wikipedia.org/wiki/Selection_algorithm
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*
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* @author Ziver
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*
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*/
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@SuppressWarnings("unchecked")
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public class QuickSelect {
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public static Object find(SortableDataList list, int k){
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return find(list, k, 0, list.size()-1);
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}
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/*
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function select(list, k, left, right)
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select a pivot value list[pivotIndex]
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pivotNewIndex := partition(list, left, right, pivotIndex)
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if k = pivotNewIndex
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return list[k]
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else if k < pivotNewIndex
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return select(list, k, left, pivotNewIndex-1)
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else
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return select(list, k, pivotNewIndex+1, right)
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*/
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public static Object find(SortableDataList list, int k, int left, int right){
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// select a pivot
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int pivot = right/2;
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int newPivot = partition(list, left, right, pivot);
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if(k == newPivot)
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return list.getIndex(k);
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else if(k < newPivot)
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return find(list, k, left, newPivot-1);
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else
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return find(list, k, newPivot+1, right);
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}
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/*
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function partition(list, left, right, pivotIndex)
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pivotValue := list[pivotIndex]
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swap list[pivotIndex] and list[right] // Move pivot to end
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storeIndex := left
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for i from left to right-1
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if list[i] < pivotValue
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swap list[storeIndex] and list[i]
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storeIndex := storeIndex + 1
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swap list[right] and list[storeIndex] // Move pivot to its final place
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return storeIndex
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*/
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private static int partition(SortableDataList list, int left, int right, int pivot){
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Object pivotValue = list.getIndex(pivot);
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list.swap(pivot, right);
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int storeIndex = left;
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for(int i=left; i<right ;i++){
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if(list.compare(i, pivotValue) < 0){
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list.swap(storeIndex, i);
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storeIndex = storeIndex+1;
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}
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}
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list.swap(right, storeIndex);
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return storeIndex;
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}
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}
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