Isaac Fei — NotesA learning log. Tagged as I go, not finished articles.https://isaacfei.comI thought four-peg Hanoi was easyhttps://isaacfei.com/notes/strange-hanoihttps://isaacfei.com/notes/strange-hanoiThree-peg Hanoi is 2 T_3(n-1)+1. Four pegs I thought was the same idea. I looked it up. One extra peg, decades to prove.Fri, 11 Sep 2026 00:00:00 GMTThe first row kills ithttps://isaacfei.com/notes/flip-switchhttps://isaacfei.com/notes/flip-switchThe action space is finite: one press per cell. Sequences of actions look huge. Two properties cut the plans to 2^{MN}. Then the first row makes the rest forced: 2^N.Wed, 09 Sep 2026 00:00:00 GMTPeel the ones, then 2^k mod 37https://isaacfei.com/notes/find-bit-positionshttps://isaacfei.com/notes/find-bit-positionslowbit walks every 1. A 37-slot table names the exponent, because 2^k is unique mod 37.Mon, 07 Sep 2026 00:00:00 GMTlowbit is n & -nhttps://isaacfei.com/notes/lowbithttps://isaacfei.com/notes/lowbitFlip the bits, add one, AND: the lowest set bit is n & -n.Sun, 06 Sep 2026 00:00:00 GMTI wanted that fun backhttps://isaacfei.com/notes/starting-algorithmshttps://isaacfei.com/notes/starting-algorithmsIn junior high I refused the editorial and could not sleep, and it still was not painful. Then CLRS for exams, LeetCode for jobs. Now I have some math. I want that fun back.Fri, 04 Sep 2026 00:00:00 GMT