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$ curl repovive.com/roadmaps/fundamental-algorithms/two-pointers/fast-and-slow-pointers

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$ curl repovive.com/roadmaps/fundamental-algorithms/two-pointers/fast-and-slow-pointers

Repovive

Fast and Slow Pointers - Two Pointers | Fundamental Algorithms | Repovive

Repovive.

Fundamental Algorithms8 sections · 294 units2h total

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Fast and Slow Pointers

Detect cycles and find midpoints.

Fast/slow pointers (Floyd's): fast moves $2\times$ slow's speed.

Cycle detection:

slow = fast = head
while fast and fast.next:
    slow = slow.next
    fast = fast.next.next
    if slow == fast: return true
return false

Why it works: In a cycle, fast laps slow. Distance decreases by $1$ each step.

Middle of list: When fast reaches end, slow is at middle.

Time: $O(n)$ . Space: $O(1)$ .