O(1) TREE

There is a tree with $N$ vertices. The vertices are numbered 1 through $N$. For each $1≤i≤N−1$, the $i-th$ edge connects vertices $ai$ and $bi$. The lengths of all the edges are 1. Chad likes some of the vertices. The information on his favorite vertices are given to you as a string s of length $N$. For each $1≤i≤N$, si is 1 if Chad likes vertex i, and 0 if he does not like vertex i. Initially, al