Recursion coding problems

Recursion is the base of backtracking, trees and divide and conquer. Practise writing the base case first and trusting the smaller call.

22 problems: 11 easy, 10 medium, 1 hard. Free to read; a free account solves them and earns XP and Aura.

1. Count DigitsPractice · +10 XP · recursion, matheasy
2. Count Paths on a LinePractice · +10 XP · dynamic programming, recursion, countingeasy
3. Count Subsequences Summing to KPractice · +10 XP · dynamic programming, recursion, countingeasy
4. Print Numbers in Reverse OrderPractice · +10 XP · recursion, arrays, iterationeasy
5. Recursive FactorialPractice · +10 XP · recursion, matheasy
6. Reverse ArrayPractice · +10 XP · recursion, arrays, backtrackingeasy
7. Reverse Number DigitsPractice · +10 XP · recursion, integerseasy
8. Root to Leaf Path SumPractice · +10 XP · trees, traversals, recursioneasy
9. Sum of Array ElementsPractice · +10 XP · recursion, arrays, iterationeasy
10. Sum of DigitsPractice · +10 XP · recursion, arithmeticeasy
11. Sum of Subtree SizesPractice · +10 XP · trees, dfs, recursioneasy
12. Balanced Parentheses CheckPractice · +20 XP · recursion, strings, parsingmedium
13. Count Pairs with Equal XOR SumPractice · +20 XP · recursion, prefix sums, hash mapsmedium
14. Find Minimum Depth Binary TreePractice · +20 XP · recursion, trees, bfsmedium
15. Lexicographically Smallest PathPractice · +20 XP · recursion, dynamic programming, graphsmedium
16. Longest Zigzag SubsequencePractice · +20 XP · recursion, dynamic programmingmedium
17. Maximum Sum Increasing SubsequencePractice · +20 XP · recursion, dynamic programming, binary indexed treemedium
18. Maximum Sum Non-Adjacent PairsPractice · +20 XP · dynamic programming, recursion, memoizationmedium
19. Subsegment Sum Divisible by KPractice · +20 XP · recursion, prefix sums, hashingmedium
20. Sum of Distinct ElementsPractice · +20 XP · recursion, hashing, arraysmedium
21. Sum of Two Adjacent ElementsPractice · +20 XP · recursion, sliding window, prefix summedium
22. Longest Increasing Subsequence on a PermutationPractice · +30 XP · recursion, dynamic programming, greedyhard