Given array a and window size k, find the maximum sum of any contiguous k elements.
Given an array of stock prices by day, find the maximum profit from one buy-sell pair.
Given a string, find the length of the longest substring without repeating characters.
Given string s and integer k, find the longest substring where you can replace at most k characters to make all characters the same.
Given strings s1 and s2, return true if s2 contains a permutation of s1 as a substring.
Given strings s and t, find the minimum window in s that contains all characters of t.