##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
Find where the cycle begins.
Problem: Find where a linked list cycle begins.
Approach: Fast/slow pointers find meeting point in cycle.
Reset one to head, move both at same speed.
They meet at cycle start.
Math: Fast travels , slow travels . Since , we get . From meeting point, steps reaches cycle start, same as steps from head.
Time: . Space: .
