Check diagonal and compute window statistics

##### Question Given a square matrix, determine whether all elements on its main diagonal are identical. LeetCode 346. Moving Average from Data Stream – design a class that maintains a sliding window of size k over a data stream and returns the current moving average on each insertion. LeetCode 480. Sliding Window Median – for an integer array and window size k, output the median of the numbers inside the window as it slides one position at a time. https://leetcode.com/problems/moving-average-from-data-stream/ https://leetcode.com/problems/sliding-window-median/