295. Find Median from Data Stream

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LeetCode

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Problem

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

• For example, for arr = [2,3,4], the median is 3.

• For example, for arr = [2,3], the median is (2 + 3) / 2 = 2.5. Implement the MedianFinder class:

• MedianFinder() initializes the MedianFinder object.
• void addNum(int num) adds the integer num from the data stream to the data structure.
• double findMedian() returns the median of all elements so far. Answers within 10-5 of the actual answer will be accepted.

Example 1:

Input
["MedianFinder", "addNum", "addNum", "findMedian", "addNum", "findMedian"]
[[], [1], [2], [], [3], []]
Output
[null, null, null, 1.5, null, 2.0]

Explanation
MedianFinder medianFinder = new MedianFinder();
medianFinder.addNum(1);    // arr = [1]
medianFinder.addNum(2);    // arr = [1, 2]
medianFinder.findMedian(); // return 1.5 (i.e., (1 + 2) / 2)
medianFinder.addNum(3);    // arr[1, 2, 3]
medianFinder.findMedian(); // return 2.0

Code

class MedianFinder {
    /** initialize your data structure here. */
    PriorityQueue<Integer> small;
    PriorityQueue<Integer> large;
    public MedianFinder() {
        small = new PriorityQueue<>();
        large = new PriorityQueue<>();
    }
    
    public void addNum(int num) {
        small.add(-num);
        if(small.size() > large.size()){
            large.offer(-small.poll());
        } else {
            // 新放进去small的数字有可能是更大的数字，而不是中间的
            // small -1
            // large 2, 3
            // 此时加入4
            // small -4, -1
            // large 2, 3
            // 此时需要重新排序
            large.offer(-small.poll());
            small.offer(-large.poll());
        }
    }
    
    public double findMedian() {
        if(small.size() != large.size()){
            return large.peek();
        } else {
            return ((double)large.peek() - (double)small.peek()) / 2;
        }
    }
}

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