题目：
题解：
class Solution {public double[] medianSlidingWindow(int[] nums, int k) {DualHeap dh = new DualHeap(k);for (int i = 0; i < k; ++i) {dh.insert(nums[i]);}double[] ans = new double[nums.length - k + 1];ans[0] = dh.getMedian();for (int i = k; i < nums.length; ++i) {dh.insert(nums[i]);dh.erase(nums[i - k]);ans[i - k + 1] = dh.getMedian();}return ans;}
}class DualHeap {// 大根堆，维护较小的一半元素private PriorityQueue<Integer> small;// 小根堆，维护较大的一半元素private PriorityQueue<Integer> large;// 哈希表，记录「延迟删除」的元素，key 为元素，value 为需要删除的次数private Map<Integer, Integer> delayed;private int k;// small 和 large 当前包含的元素个数，需要扣除被「延迟删除」的元素private int smallSize, largeSize;public DualHeap(int k) {this.small = new PriorityQueue<Integer>(new Comparator<Integer>() {public int compare(Integer num1, Integer num2) {return num2.compareTo(num1);}});this.large = new PriorityQueue<Integer>(new Comparator<Integer>() {public int compare(Integer num1, Integer num2) {return num1.compareTo(num2);}});this.delayed = new HashMap<Integer, Integer>();this.k = k;this.smallSize = 0;this.largeSize = 0;}public double getMedian() {return (k & 1) == 1 ? small.peek() : ((double) small.peek() + large.peek()) / 2;}public void insert(int num) {if (small.isEmpty() || num <= small.peek()) {small.offer(num);++smallSize;} else {large.offer(num);++largeSize;}makeBalance();}public void erase(int num) {delayed.put(num, delayed.getOrDefault(num, 0) + 1);if (num <= small.peek()) {--smallSize;if (num == small.peek()) {prune(small);}} else {--largeSize;if (num == large.peek()) {prune(large);}}makeBalance();}// 不断地弹出 heap 的堆顶元素，并且更新哈希表private void prune(PriorityQueue<Integer> heap) {while (!heap.isEmpty()) {int num = heap.peek();if (delayed.containsKey(num)) {delayed.put(num, delayed.get(num) - 1);if (delayed.get(num) == 0) {delayed.remove(num);}heap.poll();} else {break;}}}// 调整 small 和 large 中的元素个数，使得二者的元素个数满足要求private void makeBalance() {if (smallSize > largeSize + 1) {// small 比 large 元素多 2 个large.offer(small.poll());--smallSize;++largeSize;// small 堆顶元素被移除，需要进行 pruneprune(small);} else if (smallSize < largeSize) {// large 比 small 元素多 1 个small.offer(large.poll());++smallSize;--largeSize;// large 堆顶元素被移除，需要进行 pruneprune(large);}}
}
