What is the time complexity of floyd's cycle detection?

Worst case O(n). Space complexity is O(1).

Floyd's cycle detection

medium

The fast pointer gains one position per step on the slow one, so they must meet. Then restarting one at the head finds the loop entry - and the algebra behind that is worth seeing once.

Worst O(n)Space O(1)

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Where to go next

Reverse a linked list

medium

Three pointers, and the order of four lines is everything.

Circular linked list

medium

The tail points back at the head.

Two pointers

easy

One comparison eliminates a whole row of the brute force.