Simple Java Solution with 2 Heaps and Explanation

  • 10
        class MedianFinder {
        PriorityQueue<Integer> min = null;
        PriorityQueue<Integer> max = null;
        int len = 0;
        public MedianFinder(){
            min = new PriorityQueue<>();
            max = new PriorityQueue<>(Collections.reverseOrder());
        //Understanding is we will be maintain 2 heaps max and min. The max heap will store the smaller half
        //and the min heap will store the larger half of the incoming numbers. Basically we are trying to mimic a 
        //balanced tree with this model of representation, which means the max heap at the most can have only one extra
        //element than the min heap. We need to maintain the ordering that the root of the max heap < root of min heap
        // Adds a number into the data structure.
        public void addNum(int num) {
            //if the number of elements is even then we need to add the new element to the max heap
                //if the number of elements is odd then we need to add the new element to the maxheap
                //then transfer the root of the max heap to the min heap
            //increment the size
            //if the root of max heap > root of min heap, then it conficts our order rule
            //so we shift the max heap's root to min heap and add the minimum element of our
            //min heap to max heap
            if(len > 1 && max.peek() > min.peek()){
        // Returns the median of current data stream
        public double findMedian() {
            return (len%2==0)?((double)((max.peek()+min.peek())/2.0)):((double)max.peek());

  • 0

    Actually you don't need to move the element from max heap to min heap when len is odd in addNum(), just add num to the min heap. If it's smaller than the root of max heap, subsequent operations will resolve this conflict.

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