20260131

rest day

LeetCode
- 1929. Concatenation of Array (code review)
- 1480. Running Sum of 1d Array (code review)

20260130

full-time job
LeetCode
- 268. Missing Number (interview)
- 322. Coin Change (code review)
1 hour walk with my wife, arm curl, push ups

20260129

full-time job
LeetCode
- 56. Merge Intervals
- 46. Permutations (code review)
- 981. Time Based Key-Value Store
  - PR: https://github.com/huyfififi/coding-challenges/pull/47
vehicle registration renewal

20260128

full-time job (catching up with the speed slowly…)
- I’m proud of me, processing that amount of information in one day
LeetCode
- 102. Binary Tree Level Order Traversal (code review)

20260123-20260127

infected with singles, severe headache, severe nausea, fever, sickness

I thought i would die because I could not eat/drink anything for almost three days (I vomitted instantly when I tried to ingest something)

20260122

git rebase --onto main leetcode-236-lowest-common-ancestor-of-a-binary-tree

20 minutes walk
full-time job
LeetCode
- 981. Time Based Key-Value Store

20260121

pain, fatigue

took my wife to the test center
full-time job
LeetCode
- 1. Two Sum (code review)
- 141. Linked List Cycle (code review)
- 236. Lowest Common Ancestor of a Binary Tree
  - PR: https://github.com/huyfififi/coding-challenges/pull/46

20260120

fatigue (?)

full-time job (review, MTG coordination)
1 hour walk, arm curl, 30 minutes run
LeetCode
- 111. Minimum Depth of Binary Tree
  - recursive approach, BFS

20260119

25 minutes walk, 50 minutes walk, arm curl, push ups
code review (full-time job)
LeetCode
- 104. Maximum Depth of Binary Tree (code review + did it myself)
  - std::stack<std::pair<const TreeNode*, int>> node_and_depths;
  - struct
- 56. Merge Intervals
  - PR: https://github.com/huyfififi/coding-challenges/pull/45
picture drawing

20260118

returned a car to my parents
called a car insurance agent
lost my wallet and requested card replacements (and some effort to find the lost wallet)
LeetCode
- 127. Word Ladder
- 56. Merge Intervals
1 hour run

20260117

2 hours nap
25 minutes walk, 30 minutes walk, 1 hour walk
LeetCode
- 1929. Concatenation of Array (code review)
- 323. Number of Connected Components in an Undirected Graph (code review)
- 46. Permutations
  - considered suggested approaches
    - flag array
    - prefix_and_remaining

20260116

I will turn off the alarm for tomorrow…

brought my car to home
full-time job
- unexpectedly hard
LeetCode
- 695. Max Are of Islands (code review)
push ups, 1 hour jog

20260115

full-time job
LeetCode
- 39. Combination Sum (code review)
- 200. Number of Islands (code review)
20 minutes walk, push ups, arm curl, 20 minutes jog

20260114

full-time job
LeetCode
- 46. Permutations
- 200 . Number of Islands (code review)
social events
- Chichen Manchurian was good
30 minutes walk, 35 minutes run

20260113

40 minutes walk, push ups, 30 minutes run
full-time job
LeetCode 39. Combination Sum
- PR: https://github.com/huyfififi/coding-challenges/pull/43
LeetCode 46. Permutations
- Classic problem but it is hard to reason time/space complexity
LeetCode 560. Subarray Sum Equals K (code review)
- TIL: one way hash map solution
scheduled appointments

20260112

grep -E '^- \[.*\]\(https://github.com/.*/pull/[0-9]+\)' leetcode.md | wc -l

LeetCode 929. Unique Email Addresses (code review)
- std::string::substr, std::string::erase, std::set::emplace
LeetCode 112. Path Sum (code review)
full-time job
30 minutes walk
LeetCode 98. Validate Binary Search Tree (code review)
The Technological Republic
- rudder, supersede, opaque, prod, pointy, maturation, quintessential, plausible
40 minutes walk + English conversation with ChatGPT
LeetCode 39. Combination Sum
LeetCode 387. First Unique Character in a String (code review)

20260111

I was thinking about just chilling today, but I ended up doing many household chores and studying hard.

30 minutes walk
LeetCode 349. Intersection of Two Arrays (code review)
- pruning elements to reduce the number of loops
LeetCode 108. Convert Sorted Array to Binary Search Tree (code review)
LeetCode 33. Search in Rotated Sorted Array
- PR: https://github.com/huyfififi/coding-challenges/pull/42
40 minutes run

20260110

LeetCode 49. Group Anagrams
- code review (TIL: std::move)
LeetCode 994. Rotting Oranges
- PR: https://github.com/huyfififi/coding-challenges/pull/41
40 minutes walk + English conversation with ChatGPT
LeetCode Weekly Contest 484
push ups, 40 minutes walk

20260109

full-time job
LeetCode 617. Merge Two Binary Trees
- code review (newline at the end of file, the scope of helper function, Command-query separation)
LeetCode 1. Two Sum
- code review (big O notation)
Hinodeya Ramen and Dr. Ink w/ my wife
LeetCode 200. Number of Islands
LeetCode 207. Course Schedule
LeetCode 133. Clone Graph
- I was surprised I could reproduce solutions for 207 and 133. It is good.
30 minutes walk
LeetCode 200. Number of Islands
- PR: https://github.com/huyfififi/coding-challenges/pull/40
30 minutes run

20260108

full-time job
LeetCode 373. Find K Pairs with Smallest Sum
- tried myself
- code review
- There are two approaches
  - starting from (i, 0), check (index1, index2 + 1)
  - starting from (0, 0), check (index1 + 1) and (index1, index2 + 1)
30 minutes walk
30 minutes walk + English conversation with ChatGPT
LeetCode 98. Validate Binary Search Tree
- PR: https://github.com/huyfififi/coding-challenges/pull/39

import heapq


class Solution:
    def kSmallestPairs(
        self, nums1: list[int], nums2: list[int], k: int
    ) -> list[list[int]]:
        sum_and_pairs: list[tuple[int, tuple[int, int]]] = [
            (nums1[0] + nums2[0], (0, 0))
        ]
        checked_pairs: set[tuple[int, int]] = set()
        smallest_pairs: list[list[int]] = []
        while sum_and_pairs and len(smallest_pairs) < k:
            index1, index2 = heapq.heappop(sum_and_pairs)[1]
            if (index1, index2) in checked_pairs:
                continue

            smallest_pairs.append([nums1[index1], nums2[index2]])
            checked_pairs.add((index1, index2))

            if index1 + 1 < len(nums1):
                heapq.heappush(
                    sum_and_pairs,
                    (nums1[index1 + 1] + nums2[index2], (index1 + 1, index2)),
                )
            if index2 + 1 < len(nums2):
                heapq.heappush(
                    sum_and_pairs,
                    (nums1[index1] + nums2[index2 + 1], (index1, index2 + 1)),
                )

        return smallest_pairs

import heapq


class Solution:
    def kSmallestPairs(
        self, nums1: list[int], nums2: list[int], k: int
    ) -> list[list[int]]:
        sum_and_pairs = []
        for i in range(len(nums1)):
            heapq.heappush(sum_and_pairs, (nums1[i] + nums2[0], (i, 0)))

        smallest_pairs = []
        while sum_and_pairs and len(smallest_pairs) < k:
            index1, index2 = heapq.heappop(sum_and_pairs)[1]
            smallest_pairs.append([nums1[index1], nums2[index2]])

            if index2 + 1 < len(nums2):
                heapq.heappush(
                    sum_and_pairs,
                    (nums1[index1] + nums2[index2 + 1], (index1, index2 + 1)),
                )

        return smallest_pairs

20260107

full-time job
reviewed English words I noted yesterday
- engross, induce, condone, innocuous, vagary, sate
squat, push ups
The Technological Republic
- supersede, fetter, unfettered, corporeal
LeetCode 238. Product of Array Except Self
- checked review comments by Noda-san
  - rbegin(), rend()
  - std::partial_sum() in numeric
  - std::multiplies is in functional
LeetCode 347. Top K Frequent Elements
- tried heap and sort
- code review
LeetCode 155. Min Stack
- PR: https://github.com/huyfififi/coding-challenges/pull/38
push ups, 20 minutes run

20260106

LeetCode 322. Coin Change
- left comments on my own PR, discussing the program’s processing time
LeetCode 104. Maximum Depth of Binary Tree
- code review (time/space complexity, simpler condition branching)
push ups
full-time job
Barron’s Essential Words for the TOEFL
- worthwhile, ascertain, designate. brash, engross, precipitate, oblivious, encompass, inundate, impartial, appallingly, brazen, induce, teem (be teeming with), potent, apt (appropriate, be apt to), aggravate
The Technological Republic
- ascendant, condone, innocuous, vagary, sate
50 minutes walk + English conversation with ChatGPT
LeetCode 238. Product of Array Except Self
25 minutes run

20260105

full-time job
- checked all the updates while I was away
- did some bug fixes
LeetCode 703. Kth Largest Element in a Stream
- code review (name mismatch between a constructor and functions)
- for (auto it : num_to_count) { iterating std::map -> it.first, it.second
1 hour walk / English conversation with ChatGPT
LeetCode 322. Coin Change
- tabulation / memoization
- PR: https://github.com/huyfififi/coding-challenges/pull/36

20260104

replied to Nishimura-san
LeetCode 206. Reverse Linked List
- code review (variable names do not reflect their values/meanings)
- it was unexpectedly hard to write the solution…
replied to Noda-san’s comment
LeetCode 207. Course Schedule (Kahn’s algorithm, DFS)
LeetCode 208. Implement Trie
- C++ memory leak (free memory or use std::unique_ptr / std::shared_ptr)
- PR: https://github.com/huyfififi/coding-challenges/pull/35
sorted out Dependabot PRs (full-time position)
30 minutes jog

20260103

LeetCode 20. Valid Parentheses
- checked review comments
  - std::stack.empty() over std::stack.size() == 0
  - for historical reasons, when writing if condition, “…curly braces may be omitted if as a result the entire statement appears on either a single line”)
LeetCode 207. Course Schedule
- checked review comments
  - received feedback from Noda-san here: https://github.com/huyfififi/coding-challenges/pull/34
  - I was forgetting that dict keys can be traversed without dict.keys()
  - C++: array cannot be created with a size that depends on run time execution
  - retried Kahn’s algorithm
  - tackled the vector-based approach (replace std::set -> std::vector by defining num class constants)
- LeetCode went down :sweat:
30 minutes walk, push ups
leetCode 21. Merge Two Sorted Lists
- “Use values to own data, pointers to traverse linked structures, and references to alias a specific object.” by ChatGPT
- list1 -> address of node, *list1 -> the node itself
- Every new must have a corresponding delete.
LeetCode 122. Best Time to Buy and Sell Stock II
- 1 -> 5 is equivalent to 1 -> 3 and 3 -> 5 (greedy)
- complicated DP

#include <algorithm>
#include <vector>

class Solution {
public:
    int maxProfit(std::vector<int>& prices) {
        int profit = 0;
        for (int i = 1; i < prices.size(); ++i) {
            profit += std::max(0, prices[i] - prices[i - 1]);
        }
        return profit;
    }
};

20260102

LeetCode 207. Course Schedule
- A closure = a function + the variables it captures from outside
- in most cases, simple DFS or BFS is enough
  - English grammar: A or B → verb agrees with the nearest subject
- DFS
- Kahn’s algorithm (BFS)
- PR: https://github.com/huyfififi/coding-challenges/pull/34
LeetCode 20. Valid Parentheses
- code review (dangling else, No newline at end of file, return bool, map variable name)
30 minutes jog

20260101

LeetCode 1. Two Sum
- double for loops
- std::map
- std::sort, auto [variable1, variable 2]
- std::vector can be passed by value in addition to by reference, but the most common pattern is const + reference as it can block modifications + eliminate the need of copying values
The Technological Republic
- spur, spurn, untangle, disentangle, electorate, unmoored, abandonment, textile
Computer Organization and Design
- SPEC (System Performance Evaluation Cooperative)
  - non-profit organization that creates standardized benchmarks to measure and compare computer system performance in a fair, reproducible way
  - The SPEC CPU® 2017 benchmark benchmark package contains SPEC’s next-generation, industry-standardized, CPU intensive suites for measuring and comparing compute intensive performance, stressing a system’s processor, memory subsystem and compiler.
LeetCode 207. Course Schedule
- remembering topological sort
replied to some work-related messages
push ups, 40 minutes jog

TODO:

dentist’s appointment (20260217 11:30 am)

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