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1 Reverse Nodes in k-Group Hard 2 Lowest Common Ancestor of a Binary Search Tree Medium 3 Binary Tree Maximum Path Sum Hard 4 K Closest Points to Origin Medium 5 Permutations Medium 6 Number of Islands Medium 7 Course Schedule Medium 8 Min Cost to Connect All Points Medium 9 House Robber Medium 10 Word Break Medium 11 Target Sum Medium 12 Maximum Subarray Medium 13 Valid Parenthesis String Medium 14 Rotate Image Medium 15 Detect Squares Medium 16 Reverse Integer Medium 17 Valid Sudoku Medium 18 Trapping Rain Water Hard 19 Valid Parentheses Easy 20 Binary Search Easy 21 Reverse Linked List Easy 22 Find the Duplicate Number Medium 23 Balanced Binary Tree Easy 24 Validate Binary Search Tree Medium 25 Word Search II Hard 26 Find Median from Data Stream Hard 27 Palindrome Partitioning Medium 28 Surrounded Regions Medium 29 Graph Valid Tree Medium 30 Cheapest Flights Within K Stops Medium