MedianFinder.java

package com.interview.stackqueue;

import java.util.Collections;
import java.util.PriorityQueue;

/**
 * Date 03/03/2016
 * @author Tushar Roy
 *
 * Find median in stream of numbers
 * 
 * https://leetcode.com/problems/find-median-from-data-stream/
 */
public class MedianFinder {

    PriorityQueue<Integer> minPq = new PriorityQueue<>();
    PriorityQueue<Integer> maxPq = new PriorityQueue<>();

    public MedianFinder() {
        minPq = new PriorityQueue<>();
        maxPq = new PriorityQueue<>(20, Collections.reverseOrder());
    }

    // Adds a number into the data structure.
    public void addNum(int num) {
        if (maxPq.size() == 0) {
            maxPq.add(num);
            return;
        }
        if (maxPq.size() == minPq.size()) {
            if (minPq.peek() < num) {
                maxPq.offer(minPq.poll());
                minPq.offer(num);
            } else {
                maxPq.offer(num);
            }
        } else {
            int toBeOffered = 0;
            if (num >= maxPq.peek()) {
                toBeOffered = num;
            } else {
                toBeOffered = maxPq.poll();
                maxPq.offer(num);
            }
            minPq.offer(toBeOffered);
        }
    }
    // Returns the median of current data stream
    public double findMedian() {
        if (minPq.size() == maxPq.size()) {
            return ((double)minPq.peek() + (double)maxPq.peek())/2;
        } else {
            return maxPq.peek();
        }
    }

    public static void main(String args[]) {
        MedianFinder mf = new MedianFinder();
        mf.addNum(4);
        System.out.println(mf.findMedian());
        mf.addNum(8);
        System.out.println(mf.findMedian());
        mf.addNum(2);
        System.out.println(mf.findMedian());
        mf.addNum(11);
        System.out.println(mf.findMedian());
        mf.addNum(13);
        System.out.println(mf.findMedian());
        mf.addNum(14);
        System.out.println(mf.findMedian());
        mf.addNum(-1);
        System.out.println(mf.findMedian());

    }
}