Find Median from Data Stream

Source

• leetcode: Find Median from Data Stream

Median is the middle value in an ordered integer list. If the size of the
list is even, there is no middle value. So the median is the mean of the
two middle value.

Examples: 
[2,3,4] , the median is 3

[2,3], the median is (2 + 3) / 2 = 2.5

Design a data structure that supports the following two operations:

void addNum(int num) - Add a integer number from the data stream to the
data structure.
double findMedian() - Return the median of all elements so far.
For example:

add(1)
add(2)
findMedian() -> 1.5
add(3) 
findMedian() -> 2

Java

class MedianFinder {
    // max queue is always larger or equal to min queue
    PriorityQueue<Integer> min = new PriorityQueue();
    PriorityQueue<Integer> max = new PriorityQueue(1000, Collections.reverseOrder());


    // Adds a number into the data structure.
    public void addNum(int num) {
        max.offer(num);
        min.offer(max.poll());
        if (max.size() < min.size()){
            max.offer(min.poll());
        }        
    }

    // Returns the median of current data stream
    public double findMedian() {
        if (max.size() == min.size()) return (max.peek() + min.peek()) /  2.0;
        else return max.peek();        
    }
};

// Your MedianFinder object will be instantiated and called as such:
// MedianFinder mf = new MedianFinder();
// mf.addNum(1);
// mf.findMedian();

Reference

Find Median from Data Stream