Practice Lab

Solved Problems

Problem	Pattern	Language	Difficulty	Status	Notes
Subarray Sum Equals K	Prefix sum counts	Java	Medium	Solved and tuned	Count earlier prefix sums equal to `currentPrefix - k`.
Two Sum	Hash map complement lookup	Java	Easy	Solved and tuned	One-pass map from seen value to index.
Valid Parentheses	Stack matching	Java	Easy	Solved and tuned	Closing bracket maps to expected opening bracket.
Remove All Adjacent Duplicates In String	Stack cancellation	Java	Easy	Solved and tuned	Cancel the current character with the latest kept matching character.
Remove All Adjacent Duplicates In String II	Stack with run counts	Java	Medium	Solved and tuned	Compress adjacent runs and pop when a count reaches `k`.
Daily Temperatures	Monotonic stack	Java	Medium	Solved and tuned	Store unresolved day indices until a warmer day appears.
Merge Intervals	Sort intervals + scan	Java	Medium	Solved and tuned	Sort by start, then merge overlapping ranges.
Same Tree	Recursive tree comparison	Java	Easy	Solved and tuned	Compare null cases, node value, then left and right subtrees.
Binary Tree Paths	DFS path building	Java	Easy	Solved and tuned	Carry the current root-to-node path and save it at each leaf.
Jump Game	Greedy farthest reach	Java	Medium	Solved and tuned	Track the farthest reachable index; fail when the current index is beyond it.
3Sum	Sort + fixed index + two pointers	Java	Medium	Solved after TLE analysis	Brute force `O(n^3)` timed out; optimized to `O(n^2)`.
Best Time to Buy and Sell Stock	Track minimum so far	Java	Easy	Solved and tuned	One transaction: best sell today minus cheapest prior buy.
Best Time to Buy and Sell Stock II	Greedy positive differences	Java	Medium	Solved and tuned	Rising-run scan simplified to summing every positive daily gain.
Find Pivot Index	Prefix sum balance	Java	Easy	Solved and tuned	Compare left sum with total minus left sum and current value.
Rank Transform Of An Array	Sort copy + rank map	Java	Easy	Solved and tuned	Assign ranks to distinct sorted values, then map the original order.