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 263 / 268 1200
Subtree update 252 / 256 1400
Ancestor 223 / 225 1400
Shallow 201 / 205 1500
Remove subtree 135 / 176 1600
Queries on tree 99 / 155 1900
Queries on tree 2 72 / 79 1900
Library 46 / 57 1900
Black vertices 44 / 49 2000