Java Random medianHelper(List list, int k, Random r)

Description

median Helper

License

Open Source License

Declaration

private static Integer medianHelper(List<Integer> list, int k, Random r) 

Method Source Code

//package com.java2s;
/*//from w w w  .j  ava2  s.c  o  m
 * LICENSE to be determined
 */

import java.util.*;

public class Main {
    private static Integer medianHelper(List<Integer> list, int k, Random r) {
        Integer pivot;
        int numPivots = 0;
        List<Integer> less, greater;
        ListIterator<Integer> iter;

        if (list.size() == 1) {
            return list.get(0);
        }

        less = new LinkedList<Integer>();
        greater = new LinkedList<Integer>();

        pivot = list.get(r.nextInt(list.size()));
        iter = list.listIterator();
        while (iter.hasNext()) {
            Integer d = iter.next();
            if (d < pivot) {
                less.add(d);
            } else if (pivot < d) {
                greater.add(d);
            } else {
                numPivots++;
            }
        }
        //System.err.println("size=" + list.size() + "\tk=" + k +"\tpivot=" + pivot + "\tless=" + less.size() + "\tnumPivots=" + numPivots + "\tgreater=" + greater.size());

        if (k <= less.size()) {
            return medianHelper(less, k, r);
        } else if (k <= less.size() + numPivots) {
            return pivot;
        } else {
            return medianHelper(greater, k - less.size() - numPivots, r);
        }
    }
}

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