# Interview Algorithms


Insights from Leetcode problems.

#### Finding a cycle in a linked-list

Use two runners. $$\displaystyle O(n)$$
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 $$\displaystyle n$$ times and one number appears $$\displaystyle m\gt n$$ times where $$\displaystyle gcd(n,m)==1$$, then you count the number of times each bit appears and take it mod $$\displaystyle n$$.
The remaining bits will remain $$\displaystyle m \mod n$$ times.