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

Here is a secret about coding interviews. The hard part is not writing the code. It is looking at a brand new problem and knowing which tool it wants. Almost every question is an old friend wearing a costume.
And here is the good news: you have already met all the tools. Two pointers, sliding window, hash maps, trees, heaps, dynamic programming, you built each one in this track. This lesson is only about the last skill, spotting which one a problem is quietly asking for. So let's practise exactly that.
How did you spot them? By the words in the problem. Each pattern leaves fingerprints, and once you know them, half the puzzle is already solved before you write a line.
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 pairTwo Pointers
Subarray/substring of size kFixed Sliding Window
Longest/shortest subarray with conditionVariable Sliding Window
Tree/graph traversal, shortest pathBFS
Tree/graph: all paths, cycle detectionDFS
"How many ways", "minimum cost"Dynamic Programming
"Linked list cycle", "find middle"Fast & Slow Pointers
"Top k", "kth largest"Heap
Key-value lookups, frequenciesHash 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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