[leetcode]295. Find Median from Data Stream?????????????????????

Posted 2020-11-18

?????????you   following   media   vat   balance   2.0   find   design   ??????   

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.

For example,

[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.

Example:

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

 

?????????

????????????????????????????????????(????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????)

 

?????????

????????????????????????????????????????????????????????????

???PriorityQueue?????????maxHeap, minHeap, ?????????maxHeap.size() - minHeap.size() <=1 

??????????????????maxHeap.size() = minHeap.size()  ????????????????????????????????????Heap???peek???????????????

         ?????????maxHeap.size() > minHeap.size()  ??????????????????????????????maxHeap.peek

 

???????????????????????????

[6,??????10,??????2,??????6,??????5,??????0,??????6,??????3]

  ^  ???maxHeap.isEmpty() || ???????????? < maxHeap.peek() ??? ??????maxHeap??????

[6,??????10,??????2,??????6,??????5,??????0,??????6,??????3]

          ^ ??????????????? >= maxHeap.peek() ??? ??????minHeap??????

 

[6,??????10,??????2,??????6,??????5,??????0,??????6,??????3]

                   ^   ???????????? < maxHeap.peek() ??? ??????maxHeap??????

[6,??????10,??????2,??????6,??????5,??????0,??????6,??????3]

                            ^   ??????????????? >= maxHeap.peek() ??? ??????minHeap?????????minHeap???????????????????????????sort???peek??????

 

 

[6,??????10,??????2,??????6,??????5,??????0,??????6,??????3]

                                     ^   ???????????? < maxHeap.peek() ??? ??????maxHeap??????

??????maxHeap.size() - minHeap.size()  = 2 ??????????????????

???maxHeap.peek??????minHeap??????????????????

 

......

??????????????????????????????????????????

  

?????????

 1 class MedianFinder {
 2 
 3     /** initialize your data structure here. */
 4     private PriorityQueue<Integer> _maxHeap; 
 5     private PriorityQueue<Integer>  _minHeap;
 6 
 7     public MedianFinder() {
 8         _maxHeap = new  PriorityQueue<> ((o1,o2) -> o2-o1); 
 9         _minHeap = new  PriorityQueue<> ((o1,o2) -> o1-o2); 
10     }
11     
12     public void addNum(int num) {
13         // put each integer into either maxHeap or minHeap
14         if(_maxHeap.isEmpty() || num < _maxHeap.peek()){
15             _maxHeap.add(num);
16         } else{
17             _minHeap.add(num);
18         }
19         // keep balance 
20         if(_maxHeap.size() ==_minHeap.size()+2){
21             _minHeap.add(_maxHeap.poll());
22         }
23         
24           if(_minHeap.size() ==_maxHeap.size()+1){
25             _maxHeap.add(_minHeap.poll());
26         }
27         
28     }
29     // the number of data is even or odd 
30     public double findMedian() {
31         if (_maxHeap.size() == _minHeap.size()) {
32             return (_maxHeap.peek() + _minHeap.peek()) /  2.0;
33         }else {
34             return _maxHeap.peek();
35         }
36     }       
37 }
38 
39 /**
40  * Your MedianFinder object will be instantiated and called as such:
41  * MedianFinder obj = new MedianFinder();
42  * obj.addNum(num);
43  * double param_2 = obj.findMedian();
44  */