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leetcode summary 09/05

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