516 Longest Palindromic Subsequence

• O(2^n) Brute force. If the two ends of a string are the same, then they must be included in the longest palindrome subsequence. Otherwise, both ends cannot be included in the longest palindrome subsequence.

• O(n^2) memo the result from left to right. also rewrite in dp fashion

508 Most Frequent Subtree Sum

• postOrder and check count