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📚 Curriculum (10 Units)
Unit 1: Easy Warm-Ups (Arrays, Math, Strings) 10 topics
P1. Reverse Integer (LC 7)
Reverse digits of a 32-bit integer; return 0 on overflow. Key edge case: INT_MIN and trailing zeros.
P2. Palindrome Number (LC 9)
Without converting to string. Reverse half and compare; handles negative and trailing zero.
P3. Roman to Integer (LC 13)
Subtract when smaller precedes larger (IV=4, IX=9). HashMap pattern.
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P4. Longest Common Prefix (LC 14)
Vertical scan OR divide-and-conquer. Edge case: empty array.
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P5. Remove Duplicates from Sorted Array (LC 26)
Two-pointer in-place. Returns new length.
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P6. Plus One (LC 66)
Array representation of number +1 with carry propagation. Edge case: all 9s.
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P7. Sqrt(x) (LC 69)
Integer square root via binary search. Handle overflow with long or mid*mid check.
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P8. Single Number (LC 136)
Every element twice except one. XOR all elements → answer. O(1) space.
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P9. Majority Element (LC 169)
Appears more than n/2 times. Boyer-Moore voting in O(1) space.
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P10. Happy Number (LC 202)
Sum of squares of digits, repeat. Cycle detection with HashSet or Floyd's.
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Unit 2: Two Pointers & Sliding Window 10 topics
P11. Two Sum II — Input Array Is Sorted (LC 167)
Two pointers from both ends. O(n) time, O(1) space. 1-indexed result.
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P12. 3Sum (LC 15)
Sort + two pointers for each fixed element. Skip duplicates carefully. O(n²).
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P13. Move Zeroes (LC 283)
In-place two-pointer write index. Maintain relative order of non-zeros.
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P14. Remove Duplicates from Sorted Array II (LC 80)
Allow each duplicate twice. Generalize to "k times" via comparison window.
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P15. Minimum Size Subarray Sum (LC 209)
Sliding window variable size. Shrink while sum ≥ target. O(n).
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P16. Fruit Into Baskets (LC 904)
Longest subarray with at most 2 distinct values. Sliding window with HashMap.
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P17. Permutation in String (LC 567)
Fixed-size sliding window with frequency match. O(n) after preprocessing.
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P18. Longest Repeating Character Replacement (LC 424)
Sliding window: windowLen - maxFreq ≤ k keeps it valid.
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P19. Maximum Points You Can Obtain from Cards (LC 1423)
Pick k from either end. Sliding window on "leave-behind" subarray of size n-k.
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P20. Subarrays with K Different Integers (LC 992)
Count(exactly K) = Count(at most K) - Count(at most K-1). Two sliding windows.
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Unit 3: Binary Search (Array + Answer Space) 10 topics
P21. Binary Search (LC 704)
The fundamental template. Avoid integer overflow with mid = lo + (hi-lo)/2.
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P22. Search Insert Position (LC 35)
Lower_bound pattern. Returns insertion index if not found.
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P23. First Bad Version (LC 278)
Binary search on predicate. API call minimization problem.
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P24. Find First and Last Position of Element (LC 34)
Two binary searches (lower and upper bound). Returns [-1,-1] if absent.
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P25. Find Peak Element (LC 162)
Binary search on slope direction. O(log n) even in unsorted input.
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P26. Find Minimum in Rotated Sorted Array (LC 153)
Compare mid with right-end to decide half. O(log n).
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P27. Koko Eating Bananas (LC 875)
Binary search on answer. Feasibility: hours at speed k ≤ H.
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P28. Capacity to Ship Packages Within D Days (LC 1011)
Binary search on capacity. Lower bound = max weight; upper = sum.
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P29. Split Array Largest Sum (LC 410)
Binary search on max-sum per partition. Feasibility via greedy counting.
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P30. Median of Two Sorted Arrays (LC 4)
Binary search partition on smaller array. O(log(min(n,m))).
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Unit 4: Linked Lists & Iterators 10 topics
P31. Remove Duplicates from Sorted List (LC 83)
Single pass skipping duplicates. Delete current.next when values equal.
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P32. Delete Node in a Linked List (LC 237)
Only node reference given. Copy next's value then delete next.
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P33. Reverse Linked List II (LC 92)
Reverse between positions left and right. Dummy + in-place insertion.
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P34. Intersection of Two Linked Lists (LC 160)
Two pointers; swap heads at end. O(1) space.
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P35. Partition List (LC 86)
Rearrange < x before ≥ x, preserving order. Two dummy lists then join.
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P36. Design Linked List (LC 707)
Implement get, addAtHead, addAtTail, addAtIndex, deleteAtIndex. Production-style.
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P37. Flatten a Multilevel Doubly Linked List (LC 430)
DFS/stack flatten child lists inline. Update prev/next/child pointers.
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P38. Binary Search Tree Iterator (LC 173)
Controlled inorder traversal. Stack storing leftmost path; O(h) space.
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P39. Peeking Iterator (LC 284)
Wrap iterator with single-element lookahead. Design pattern.
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P40. Flatten Nested List Iterator (LC 341)
Stack-based. Push nested lists in reverse so top is next integer.
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Unit 5: Trees, BSTs & Traversals 10 topics
P41. Same Tree (LC 100)
Recursive structural + value equality check.
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P42. Symmetric Tree (LC 101)
Mirror recursion: left.left vs right.right, left.right vs right.left.
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P43. Path Sum (LC 112)
Root-to-leaf sum equals target. Recursive subtraction.
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P44. Sum Root to Leaf Numbers (LC 129)
Accumulate number * 10 + node.val down the path; add at leaves.
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P45. Balanced Binary Tree (LC 110)
Height-check returning -1 on imbalance for early exit. O(n).
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P46. Binary Tree Zigzag Level Order (LC 103)
BFS; reverse alternate levels. Or use Deque addFirst/addLast.
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P47. Lowest Common Ancestor of BST (LC 235)
Use BST ordering: split point where p and q diverge.
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P48. Count Complete Tree Nodes (LC 222)
O(log² n): check left vs right subtree depths; use formula 2^h - 1.
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P49. All Nodes Distance K in Binary Tree (LC 863)
Build parent map + BFS from target node K levels.
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P50. Binary Tree Right Side View (LC 199)
BFS: last node at each level. Or DFS visiting right first, tracking depth.
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Unit 6: Greedy & Intervals 10 topics
P51. Assign Cookies (LC 455)
Sort both; greedy two-pointer pair largest kid with smallest sufficient cookie.
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P52. Lemonade Change (LC 860)
Simulate $5, $10, $20 bills. Greedy: always give $10 before two $5s.
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P53. Jump Game (LC 55)
Track farthest reachable; return false if i exceeds it.
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P54. Jump Game II (LC 45)
Min jumps via BFS-like greedy: track currentEnd and farthest.
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P55. Gas Station (LC 134)
If total gas ≥ total cost, answer exists. Reset start on deficit.
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P56. Candy (LC 135)
Two passes: left-to-right then right-to-left, take max. O(n).
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P57. Merge Intervals (LC 56)
Sort by start; merge when current.end ≥ next.start.
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P58. Insert Interval (LC 57)
Three phases: strictly before, overlapping merge, strictly after.
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P59. Non-overlapping Intervals (LC 435)
Sort by end; keep interval with earliest end of compatible.
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P60. Minimum Arrows to Burst Balloons (LC 452)
Sort by end; shoot at earliest end of overlapping group.
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Unit 7: Dynamic Programming (1D & 2D) 10 topics
P61. Fibonacci Number (LC 509)
Intro to DP. Memoized recursion → bottom-up → O(1) space.
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P62. Climbing Stairs (LC 70)
Fibonacci equivalent. dp[n]=dp[n-1]+dp[n-2].
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P63. Min Cost Climbing Stairs (LC 746)
Take one or two steps; minimize cost. O(1) space.
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P64. Maximum Subarray (LC 53)
Kadane. Running sum; reset when negative.
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P65. Unique Paths (LC 62)
Grid DP or math combination C(m+n-2, m-1).
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P66. Minimum Path Sum (LC 64)
dp[i][j] = grid[i][j] + min(dp[i-1][j], dp[i][j-1]).
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P67. Longest Increasing Subsequence (LC 300)
O(n²) DP → O(n log n) patience sort with binary search.
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P68. Longest Common Subsequence (LC 1143)
2D DP. Space optimizable to O(min(n,m)).
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P69. Edit Distance (LC 72)
Levenshtein. Three operations; space-optimize to 1D row.
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P70. Partition Equal Subset Sum (LC 416)
Subset sum = total/2. Boolean DP, 1D space-optimized.
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Unit 8: Bit Manipulation & Number Theory 10 topics
P71. Number of 1 Bits (LC 191)
Brian Kernighan's: n &= n-1 clears lowest set bit. O(# set bits).
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P72. Reverse Bits (LC 190)
Bit-by-bit, or byte-reverse with lookup. 32-bit integer.
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P73. Counting Bits (LC 338)
dp[i] = dp[i >> 1] + (i & 1). O(n).
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P74. Power of Two (LC 231)
n > 0 && (n & (n-1)) == 0.
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P75. Missing Number (LC 268)
XOR all numbers + all indices; OR sum formula.
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P76. Single Number II (LC 137)
Every number 3x except one. Bit counting mod 3 OR two-variable state machine.
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P77. Single Number III (LC 260)
Two singletons. XOR all → group by a bit that differs → XOR each group.
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P78. Sum of Two Integers without + (LC 371)
XOR = sum without carry; AND
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P79. Divide Two Integers without / or * (LC 29)
Bit-shift doubling of divisor. Watch overflow (INT_MIN / -1).
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P80. Power(x, n) (LC 50)
Fast exponentiation by squaring. O(log |n|). Handle negative n carefully.
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Unit 9: HackerRank Favourites (Strings + Warmup + Sort) 10 topics
P81. Sock Merchant (HR Warmup)
Count pairs of matching socks. HashMap or frequency array; sum count/2.
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P82. Counting Valleys (HR Warmup)
Track sea level; count transitions into below-sea-level valleys.
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P83. Jumping on the Clouds (HR Warmup)
Greedy: jump 2 if possible, else 1. O(n).
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P84. Repeated String (HR Warmup)
Count "a"s in one period * full periods + "a"s in remainder prefix.
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P85. New Year Chaos (HR Arrays)
Count minimum bribes; return "Too chaotic" if anyone bribed > 2.
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P86. Minimum Swaps 2 (HR Arrays)
Place each value at its index; count cycle-based swaps.
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P87. Array Manipulation (HR Medium)
Difference array technique; O(n+m) instead of O(n*m).
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P88. Sherlock and Anagrams (HR Strings)
Count anagram pairs among all substrings. Sort key in HashMap.
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P89. Sparse Arrays (HR Strings)
For each query word, count occurrences. HashMap of string frequencies.
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P90. Pairs (HR HashMap)
Count pairs with difference k. HashSet lookup for (x + k) and (x - k).
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Unit 10: Hard LeetCode + Competitive Flavour 10 topics
P91. Trapping Rain Water (LC 42)
Two-pointer O(n) O(1), or monotonic stack, or DP arrays. All three walks.
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P92. Largest Rectangle in Histogram (LC 84)
Monotonic stack O(n). Sentinel 0 at end simplifies code.
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P93. Sliding Window Maximum (LC 239)
Monotonic deque of indices. O(n) with amortized analysis.
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P94. Longest Consecutive Sequence (LC 128)
O(n) with HashSet; only start counting from sequence beginnings.
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P95. LRU Cache (LC 146)
HashMap + DoublyLinkedList. O(1) get/put.
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P96. Find Median from Data Stream (LC 295)
Two heaps with size invariant. O(log n) add, O(1) findMedian.
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P97. Serialize and Deserialize BST (LC 449)
Preorder serialization; reconstruct using BST property (no null markers needed).
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P98. Word Ladder II (LC 126)
BFS + backtracking. Build parents map during BFS, DFS from end to start.
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P99. Regular Expression Matching (LC 10)
2D DP over (s, p). Star handles "zero or more of prev char".
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P100. Final Mock Assessment + Placement Playbook
Full timed mock: 3 problems in 90 minutes. HackerRank-style scoring + interview tips for service-cos vs product-cos.
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Duration
90 Days
Format
Both Classes
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