Module Euler tour

Euler tour

**Frequency: 7/10** Also known as DFS order. This is important for solving tree related problems from now on.

Resources

- [USACO: Euler Tour](https://usaco.guide/gold/tree-euler)

Problems

Subtree queries 166 / 168 1200
Subtree update 158 / 162 1400
Ancestor 141 / 143 1400
Remove subtree 85 / 111 1500
Shallow 125 / 128 1500
Queries on tree 66 / 94 1900
Queries on tree 2 47 / 54 1900
Library 28 / 36 1900
Black vertices 25 / 28 2000