Solve tree, graph, sliding-window problems

##### Question LeetCode 103. Binary Tree Zigzag Level Order Traversal – given a binary tree root, return its zigzag level-order traversal. LeetCode 207. Course Schedule – given course prerequisites, determine if all courses can be finished by detecting cycles with topological sort. LeetCode 340. Longest Substring with At Most K Distinct Characters – given string s and integer k, return the length of the longest substring containing at most k distinct characters. https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/ https://leetcode.com/problems/course-schedule/description/ https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/description/