TikTok CodeSignal OA Real Questions: Last Two Problems Full Breakdown (70min)

TikTok CodeSignal OA is 70 minutes total. This post covers Q3 and Q4 — the two questions that actually differentiate candidates. Q1 and Q2 are warm-ups; the real challenge starts at Q3. Real screenshots included below.

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Exam Overview

Item	Details
Platform	CodeSignal
Total Time	70 minutes
Questions	4
This Article	Q3 + Q4 (last two)
Role	SWE / Backend

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Q3: Real Screenshot


Core Idea

Given a string array or integer sequence, you need to apply grouping / counting / interval merging rules to find the optimal answer. Q3 on CodeSignal typically tests:

• HashMap frequency counting
• Sliding window
• Prefix sums
• Sorting + greedy

Solution Approach

Step 1: Identify the constraint

Read carefully and pin down the core constraint. Most Medium-level Q3s reduce to a known pattern once you strip the story wrapper.

Step 2: Brute force → optimize

Sketch the O(n²) brute force first, then ask: can a HashMap or two pointers bring this to O(n) or O(n log n)?

Step 3: Handle edge cases

• Empty array / empty string
• All elements identical
• Single-element input

Solution Template

def solution(arr):
    from collections import defaultdict

    count = defaultdict(int)
    result = 0

    for val in arr:
        # Query historical state before updating
        result += count[val]
        count[val] += 1

    return result

Time Complexity: O(n)
Space Complexity: O(n)

Pattern Reference

Pattern	Technique	When to Use
Pair counting	HashMap one-pass	Two Sum variants
Interval merging	Sort + linear scan	Overlapping intervals
String matching	Sliding window + Set	No-repeat substrings
Prefix counting	Prefix sum + HashMap	Subarray sum

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Q4: Real Screenshot


Core Idea

The final question is almost always the hardest — a graph traversal / state machine / complex simulation. Common topics:

• BFS / DFS graph traversal
• State machine simulation (with cycle detection)
• Dynamic programming
• Complex string processing

Solution Approach

Model the state space

1. Define what a "state" is — current position, visited nodes, remaining resources, etc.
2. Define transition rules — how you move from one state to the next.
3. Define termination conditions — success, failure (obstacle), and infinite loop.

Multi-termination state machine template

def solution(n, m, obstacles, teleports):
    obstacle_set = set(map(tuple, obstacles))
    teleport_map = {(t[0], t[1]): (t[2], t[3]) for t in teleports}

    row, col = 0, 0
    visited = set()
    steps = 0

    while True:
        state = (row, col)

        # 1. Cycle detection (must come first)
        if state in visited:
            return -2  # Infinite loop
        visited.add(state)

        # 2. Obstacle check
        if state in obstacle_set:
            return -1  # Blocked

        steps += 1

        # 3. Goal check
        if row == n - 1 and col == m - 1:
            return steps  # Reached destination

        # 4. Teleport
        if state in teleport_map:
            row, col = teleport_map[state]
            continue

        # 5. Regular move (right)
        col += 1

        # 6. Out-of-bounds check
        if col >= m:
            return -3  # Can't reach goal

    return -3

Time Complexity: O(n × m) — each cell visited at most once
Space Complexity: O(n × m) — visited set

On-site Strategy for Q4

When Q4 looks overwhelming:

1. List all termination conditions first — write them as comments before any code
2. Draw a small example — manually simulate 2-3 steps on paper
3. Start with the simplest path — get basic cases passing, then add edge cases
4. Always add cycle detection — visited set is non-negotiable

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Side-by-Side Comparison

Question	Core Technique	Time	Space	Difficulty
Q3	HashMap + single pass	O(n)	O(n)	Medium
Q4	State machine + cycle detection	O(n×m)	O(n×m)	Hard

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Common Mistakes

Question	Mistake	Correct Approach
Q3	Nested loops O(n²)	HashMap one-pass O(n)
Q3	Ignoring negative / zero edge cases	Check input range upfront
Q4	Forgetting cycle detection	Always add visited set
Q4	Wrong termination order	Check loop → obstacle → goal

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TikTok OA Strategy

CodeSignal Platform Notes

• Auto-graded; you cannot see test cases
• 70 minutes — pace yourself
• Q1/Q2 warm-up (15 min total); Q3/Q4 are the real test (55 min)
• Some companies review code quality, not just pass rate

Recommended Time Split

Question	Target Time	Strategy
Q1	5-8 min	Fast AC, don't get stuck
Q2	8-12 min	Watch edge cases
Q3	20-25 min	Think before you type
Q4	25-30 min	Write termination framework first

Related LeetCode Practice

#	Problem	Covers
1	Two Sum	Q3 HashMap
560	Subarray Sum Equals K	Q3 Prefix Sum
542	01 Matrix	Q4 BFS
200	Number of Islands	Q4 Graph Traversal
289	Game of Life	Q4 State Machine

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