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Master these coding challenges to ace your next technical interview. Handpicked problems with personalized spaced repetition.

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Title	Difficulty	Topics
1. Two Sum Open source	Easy	ArrayHash Table
2. Add Two Numbers Open source	Medium	Linked ListMathRecursion
3. Longest Substring Without Repeating Characters Open source	Medium	Hash TableStringSliding Window
4. Median of Two Sorted Arrays Open source	Hard	ArrayBinary SearchDivide and Conquer
5. Longest Palindromic Substring Open source	Medium	Two PointersStringDynamic Programming+1 more
6. Zigzag Conversion Open source	Medium	String
7. Reverse Integer Open source	Medium	Math
8. String to Integer (atoi)Open source	Medium	String
9. Palindrome Number Open source	Easy	Math
10. Regular Expression Matching Open source	Hard	StringDynamic ProgrammingRecursion
11. Container With Most Water Open source	Medium	ArrayTwo PointersGreedy
12. Integer to Roman Open source	Medium	Hash TableMathString
13. Roman to Integer Open source	Easy	Hash TableMathString
14. Longest Common Prefix Open source	Easy	ArrayStringTrie
15. 3Sum Open source	Medium	ArrayTwo PointersSorting
16. 3Sum Closest Open source	Medium	ArrayTwo PointersSorting
17. Letter Combinations of a Phone Number Open source	Medium	Hash TableStringBacktracking
18. 4Sum Open source	Medium	ArrayTwo PointersSorting
19. Remove Nth Node From End of List Open source	Medium	Linked ListTwo Pointers
20. Valid Parentheses Open source	Easy	StringStackBracket Sequences
21. Merge Two Sorted Lists Open source	Easy	Linked ListRecursion
22. Generate Parentheses Open source	Medium	StringDynamic ProgrammingBacktracking+1 more
23. Merge k Sorted Lists Open source	Hard	Linked ListDivide and ConquerHeap (Priority Queue)+2 more
24. Swap Nodes in Pairs Open source	Medium	Linked ListRecursion
25. Reverse Nodes in k-Group Open source	Hard	Linked ListRecursion