summary the following question from leetcode
0. Add summary tricks on each problem
1. BinaryTreeMaximumPathSum: tree, post-order-traverse, divide-conquer
2. App questions using Arrays. Candy: 2-ways scan
3. Combinations problem: 2^N bruce-force
4. Permutations problem: Permutations generation, and Permutations
sequences
5. Questions about math and Numbers