回复：another way to look at Quant面试题：喝醉了（2.5星）

This is a first hitting time problem of the symmetric random walk.

Assume the poor drunk guy starts from n steps far away from left door(n
since M(t) here is martingale, it's easy to show that E[tau]=n(100-n), here we have n=1, therefore E[tau]=99.