Lesson 25/25 · 🎯 Interview Patterns
🎯 Interview PatternsLesson 25/25
Phase 9 · Interview Patterns30 min

Top Interview Patterns

The 5 pattern recognition skills that unlock 80% of coding interview problems

The secret to coding interviews isn't memorizing solutions, it's pattern recognition. When you see a problem, you identify which pattern applies, then adapt the template.
The 5 most common patterns:1. Two Pointers, sorted array, pairs, palindromes2. Sliding Window, subarrays, substrings3. Fast & Slow Pointers, cycle detection, middle of list4. BFS/DFS, trees, graphs, islands5. Dynamic Programming, optimal value, count ways
Fast & Slow Pointers, detect cycle in linked listpython
class Node:
    def __init__(self, val): self.val = val; self.next = None

def has_cycle(head):
    """Floyd's algorithm: slow moves 1 step, fast moves 2."""
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow is fast:    # They met, there's a cycle
            return True
    return False            # Fast reached end, no cycle

# Build a cycle: 1→2→3→4→2 (4 points back to 2)
nodes = [Node(i) for i in range(1, 5)]
for i in range(3): nodes[i].next = nodes[i+1]
nodes[3].next = nodes[1]  # Cycle
print(has_cycle(nodes[0]))  # → True
Pattern Recognition Cheat Sheet:
| You see… | Think… ||----------|--------|| Sorted array + find pair | Two Pointers || Subarray/substring of size k | Fixed Sliding Window || Longest/shortest subarray with condition | Variable Sliding Window || Tree/graph traversal, shortest path | BFS || Tree/graph: all paths, cycle detection | DFS || "How many ways", "minimum cost" | Dynamic Programming || "Linked list cycle", "find middle" | Fast & Slow Pointers || "Top k", "kth largest" | Heap || Key-value lookups, frequencies | Hash Map |
💬In the Real World...

In real interviews, communicating your pattern recognition is as important as the solution: "I see a subarray problem with a sum condition, this looks like a variable sliding window. Let me think about the expand/contract conditions."

🤔Quick Check

A problem asks: "Find all unique triplets in an array that sum to zero." Which pattern?

🎯

Phase Complete!

Interview Patterns

You've completed the DSA & Algorithms track! You understand complexity, every major data structure, and the top interview patterns. You're interview-ready. Keep practicing!

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Practice Exercises

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Exercise 1 of 2hard
⏱ 00:00

Three Sum

Find all unique triplets that sum to zero. Use sort + two pointers.
Expected output: [[-1, -1, 2], [-1, 0, 1]]
solution.py
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Exercise 2 of 2medium
⏱ 00:00

Word Break

Given a string and a word dictionary, can the string be segmented into dictionary words?
Expected output: True
solution.py
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