🧠 7-Day Java Coding Test Rehab

코딩테스트 감각 재활을 위한 7일 집중 문제 풀이 루틴 (with LeetCode)

가장 많이 나오는 패턴 포함 (총 35문제)

• Hash / Two pointer / Sliding window / Stack / Linked list / Tree / Graph / Heap

---

🚀 Study Stack

Category	Stack
Language	Java
Platform	LeetCode
Reference	Tech Interview Handbook

---

📅 7-Day Problem List

Day 1 — Java 재활 / 기본 자료구조

Done	Problem	Link
⬜	Two Sum	LeetCode
⬜	Contains Duplicate	LeetCode
⬜	Valid Anagram	LeetCode
⬜	Valid Parentheses	LeetCode
⬜	Maximum Subarray	LeetCode
⬜	Reverse Linked List	LeetCode

Day 2 — Array / Two Pointer

Done	Problem	Link
⬜	Longest Substring Without Repeating Characters	LeetCode
⬜	Container With Most Water	LeetCode
⬜	3Sum	LeetCode
⬜	Product of Array Except Self	LeetCode
⬜	Longest Consecutive Sequence	LeetCode

Day 3 — Stack / Interval

Done	Problem	Link
⬜	Min Stack	LeetCode
⬜	Daily Temperatures	LeetCode
⬜	Merge Intervals	LeetCode
⬜	Insert Interval	LeetCode
⬜	Meeting Rooms	LeetCode

Day 4 — Linked List

Done	Problem	Link
⬜	Reverse Linked List	LeetCode
⬜	Merge Two Sorted Lists	LeetCode
⬜	Linked List Cycle	LeetCode
⬜	Remove Nth Node From End of List	LeetCode
⬜	Reorder List	LeetCode

Day 5 — Tree

Done	Problem	Link
⬜	Invert Binary Tree	LeetCode
⬜	Maximum Depth of Binary Tree	LeetCode
⬜	Validate Binary Search Tree	LeetCode
⬜	Binary Tree Level Order Traversal	LeetCode
⬜	Lowest Common Ancestor of a Binary Tree	LeetCode

Day 6 — Graph / BFS / DFS

Done	Problem	Link
⬜	Number of Islands	LeetCode
⬜	Clone Graph	LeetCode
⬜	Course Schedule	LeetCode
⬜	Pacific Atlantic Water Flow	LeetCode
⬜	Rotting Oranges	LeetCode

Day 7 — Heap / Advanced

Done	Problem	Link
⬜	Top K Frequent Elements	LeetCode
⬜	Kth Largest Element in an Array	LeetCode
⬜	Merge k Sorted Lists	LeetCode
⬜	Find Median from Data Stream	LeetCode

---

🧩 Problem Solving Strategy

1️⃣ Pattern Classification

문제를 읽고 바로 구현하지 않고 먼저 어떤 알고리즘 패턴인지 분류하기

Problem Pattern	Typical Approach
중복 검사 / 빈도 계산	HashMap / HashSet
두 값 조합 / 정렬 후 탐색	Two Pointer
부분 문자열 / 구간 최적화	Sliding Window
괄호 / 순서 / 이전 상태 비교	Stack
그래프 / 연결 구조 탐색	BFS / DFS
정렬된 데이터 탐색	Binary Search
상위 K개 문제	Heap / PriorityQueue
선후 관계 문제	Topological Sort
트리 문제	Tree DFS / BFS

2️⃣ Brute Force First

처음부터 최적해를 찾기보다는 가장 단순한 해결 방법을 먼저 생각하기

예: O(n²) brute force

• 이미 계산한 정보를 저장할 수 있을까?
  • HashMap
• 정렬하면 문제를 단순화할 수 있을까?
  • Two Pointer
• 포인터 이동으로 줄일 수 있을까?
  • Sliding Window

3️⃣ Manual Simulation

코드를 작성하기 전에 예제를 직접 시뮬레이션하기

• 문자열 문제

1. left pointer
2. right pointer
3. current substring
4. map 상태

• 그래프 문제

1. queue 상태
2. visited 상태

4️⃣ Apply Algorithm Template

패턴이 정해지면 검증된 템플릿 구조를 사용하기

Sliding Window

int left = 0;

for (int right = 0; right < n; right++) {

    // window expand

    while (condition violated) {
        left++;
    }

    // update answer
}

BFS

Queue<int[]> q = new LinkedList<>();
q.offer(start);
visited[start] = true;

while (!q.isEmpty()) {
    int cur = q.poll();
}

Two Pointer

int left = 0;
int right = arr.length - 1;

while (left < right) {

    int sum = arr[left] + arr[right];

    if (sum == target) break;

    if (sum < target)
        left++;
    else
        right--;
}

5️⃣ Edge Case Check

• empty array
• single element
• duplicate values
• negative numbers
• off-by-one (< vs <=)
• overflow 가능성