295. 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:

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

为了便于找到中位数，在添加元素时应添加到相应的位置，
使用两个优先队列来存储数据，构造大顶堆
*/

class MedianFinder {
private:
priority_queue<long> small,large;
public:
/** initialize your data structure here. */
MedianFinder() {

}

void addNum(int num) {
large.push(-num);//存放大的元素 取反后小的元素排在前面
small.push(-large.top());//大的元素排在前面
large.pop();
if(small.size()>large.size())
{
large.push(-small.top());
small.pop();
}
}

double findMedian() {
return small.size() < large.size() ? -large.top() : (small.top()-large.top())/2.0;
}
};

/**
* Your MedianFinder object will be instantiated and called as such:
* MedianFinder obj = new MedianFinder();
* obj.addNum(num);
* double param_2 = obj.findMedian();
*/