leetcode

Arrays & Hashing

Problem	Leetcode	Difficulty	Topics	Notes
217 - Contains Duplicate	Leetcode	🟢	Array, Hash Table, Sorting	⚠️ Notes (solution spoiler alert) Use dictionary (hashmap) to check for duplicates
242 - Valid Anagram	Leetcode	🟢	Hash Table, String, Sorting	⚠️ Notes (solution spoiler alert) Two dictionaries (hashmaps), char:count and then compare two hashmaps
1 - Two Sum	Leetcode	🟢	Array, Hash Table	⚠️ Notes (solution spoiler alert) TBD
49 - Group Anagrams	Leetcode	🟡	Array, Hash Table, String, Sorting	⚠️ Notes (solution spoiler alert) Use dictionary (hashmap) with sorted letters of a word (key) and array of anagrams (value)
347 - Top K Frequent Elements	Leetcode	🟡	Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect	TBD
238 - Product of Array Except Self	Leetcode	🟡	Array, Prefix Sum	⚠️ Notes (solution spoiler alert) Tricky to understand the principle. Make two passes to compute products: first in-order, second in-reverse.
36 - Valid Sudoku	Leetcode	🟡	Array, Hash Table, Matrix	⚠️ Notes (solution spoiler alert) Use 3 hashmaps (dictionaries): rows, columns and subsets (3x3, use 'row / 3', 'column / 3' as a key). Check for duplicates in sets
271 - Encode and Decode Strings	Leetcode	🟡	TBD	⚠️ Notes (solution spoiler alert) Separator is a word length + a sign (e.g., 5,3#hellohey). Loop through the counts, not through each character and append words calculating start/end indexes.
128 - Longest Consecutive Sequence	Leetcode	🟡	Array, Hash Table, Union Find	⚠️ Notes (solution spoiler alert) Loop and find out if the num is a start of a sequence (nums[i] - 1 tells that it's a lowest value of a sequence). Then calculate the length of the sequence using while and find the longest sequence.

Two Pointers

Problem	Leetcode	Difficulty	Topics	Notes
125 - Valid Palindrome	Leetcode	🟢	Two Pointers, String	⚠️ Notes (solution spoiler alert) Left and right pointer. Compare left/right chars (lowercased) in while. Increase left or decrease right pointer unless there's a letter/number (ascii) char found to compare.
167 - Two Sum II - Input Array Is Sorted	Leetcode	🟡	Array, Two Pointers, Binary Search	⚠️ Notes (solution spoiler alert) In while calculate the sum (left and right pointers). If sum > target, decrease right pointer. If sum < target, increase left pointer. If sum == target, return.
15 - 3Sum	Leetcode	🟡	Array, Two Pointers, Sorting	⚠️ Notes (solution spoiler alert) Similar to the "2 Sum" approach. Sort the array first. Skip dups while iterating. Then apply "2 Sum" method, for each num and update left pointer (while no dup value found).
11 - Container With Most Water	Leetcode	🟡	Array, Two Pointers, Greedy	⚠️ Notes (solution spoiler alert) The left p is the start, and the right p is the end of the array. Calculate the max area ((r - l) * min(h[l], h[r])) and update the left or right pointer depending on whether h[l] is < or > than h[r].
42 - Trapping Rain Water	Leetcode	🔴	Array, Two Pointers, DP, Stack, Monotonic Stack	⚠️ Notes (solution spoiler alert) TBD

Stack

Problem	Leetcode	Difficulty	Topics	Notes
20 - Valid Parentheses	Leetcode	🟢	String, Stack	⚠️ Notes (solution spoiler alert) Use simple stack structure (Character). Declare Char:Char hashmap [")": "(", "}": "{", "]": "["]. Go through the string and check if char is an open bracket (if so, push). If not, check if top char is an open bracket for char (if yes, then pop; if not, then return false).
155 - Min Stack	Leetcode	🟡	Stack, Design	⚠️ Notes (solution spoiler alert) Store two arrays (one for numbers, another for min values). push - append value to stack & append minStack value (min(val, minStack.last ?? val)). pop should pop from both. top - return last from stack, getMin - last from minStack.
150 - Evaluate Reverse Polish Notation	Leetcode	🟡	Array, Math, Stack	⚠️ Notes (solution spoiler alert) Iterate through the strings and use switch. In case of an operator, pop 2 values, calcuate the expression (mind the order) and push to the stack. In case of a number (default case), just push it to the stack. Return top value.
22 - Generate Parentheses	Leetcode	🟡	String, DP, Backtracking	⚠️ Notes (solution spoiler alert) Implement a backtracking function and call it recursively. Pass n, openN, closedN, char stack and res array. In it, check for the valid result first (n == openN == closedN), join stack, append to res and return. Push open paranthesis if open < n, backtrack (openN + 1) and pop. Push close paranthesis if closedN < openN, backtrack (closedN +1) and pop.
739 - Daily Temperatures	Leetcode	🟡	Array, Stack, Monotonic Stack	⚠️ Notes (solution spoiler alert) TBD to deep dive
853 - Car Fleet	Leetcode	🟡	Array, Stack, Sorting, Monotonic Stack	⚠️ Notes (solution spoiler alert) Notes
84 - Largest Rectangle In Histogram	Leetcode	🔴	TBD	⚠️ Notes (solution spoiler alert) TBD

Binary Search

Problem	Leetcode	Difficulty	Topics	Notes
704 - Binary Search	Leetcode	🟢	Array, Binary Search	⚠️ Notes (solution spoiler alert) TBD