Java实现用最大堆和最小堆查找中位数 Find median with min heap and max heap in Java

Google面试题

股市上一个股票的价格从开市开始是不停的变化的，需要开发一个系统，给定一个股票，它能实时显示从开市到当前时间的这个股票的价格的中位数（中值）。

import java.util.*;/** * Find median with max-heap and min-heap * O(1) find and O(logN) insert  * */public class FindMedian {private static PriorityQueue<Integer> maxHeap, minHeap;private static int numOfElements = 0;public static void main(String[] args) {Comparator<Integer> revCmp = new Comparator<Integer>() {@Overridepublic int compare(Integer left, Integer right) {return right.compareTo(left);}};// Or you can use Collections' reverseOrder method as follows.// Comparator<Integer> revCmp = Collections.reverseOrder();maxHeap = new PriorityQueue<Integer>(20, revCmp);minHeap = new PriorityQueue<Integer>(20);addNumber(6);addNumber(4);addNumber(3);addNumber(10);addNumber(12);System.out.println(minHeap);System.out.println(maxHeap);System.out.println(getMedian());addNumber(5);System.out.println(minHeap);System.out.println(maxHeap);System.out.println(getMedian());addNumber(7);addNumber(8);System.out.println(minHeap);System.out.println(maxHeap);System.out.println(getMedian());}/* * Maintains a condition that maxHeap.size() >= minHeap.size() * 1) the max-heap contains the smallest half of the numbers and min-heap contains the largest half * 2) the number of elements in max-heap is either equal to or 1 more than the number of elements in the min-heap */public static void addNumber(int value) {maxHeap.add(value);// If total number of elements in the heap is even before insertion,// then there are N elements both in max-heap and min-heap.// Insert to max-heap, result in max-heap has n+1 elements, but this is// valid since max-heap can contain 1 more element than min-heapif (numOfElements%2 == 0) {if (minHeap.isEmpty()) {numOfElements++;return;}// If the newly inserted value is larger than root of min-heap// we need to pop the root of min-heap and insert it to max-heap.// And pop root of max-heap and insert it to min-heapelse if (maxHeap.peek() > minHeap.peek()) {Integer maxHeapRoot = maxHeap.poll();Integer minHeapRoot = minHeap.poll();maxHeap.add(minHeapRoot);minHeap.add(maxHeapRoot);} } // For this case, before insertion, max-heap has n+1 and min-heap has n elements.// After insertion, max-heap has n+2 and min-heap has n elements, so violate!// And we need to pop 1 element from max-heap and push it to min-heapelse {minHeap.add(maxHeap.poll());}numOfElements++;}/* * If maxHeap and minHeap are of different sizes, then maxHeap must have one * extra element. */public static double getMedian() {if (numOfElements%2 != 0)// If total number received is not evenreturn new Double(maxHeap.peek());elsereturn (maxHeap.peek() + minHeap.peek()) / 2.0; }}

Ref:

http://www.ardendertat.com/2011/11/03/programming-interview-questions-13-median-of-integer-stream/

http://blog.sina.com.cn/s/blog_979956cc0101hab8.html

http://blog.csdn.net/ajaxhe/article/details/8734280

http://www.cnblogs.com/remlostime/archive/2012/11/09/2763256.html