Interview Algorithms

Insights from Leetcode problems.

Finding a cycle in a linked-list

Use two runners. $O(n)$
Runner 1 goes two steps per iteration.
Runner 2 goes one step per iteration.
If there is a cycle, runner 2 will lap runner 1 within 2 cycles.

Finding duplicates in an array

If you have an array of ints where each number appears $n$ times and one number appears $m>n$ times where $gcd(n,m)==1$, then you count the number of times each bit appears and take it mod $n$.
The remaining bits will remain $m\mod n$ times.