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 247 / 251 1200
Subtree update 240 / 243 1400
Ancestor 213 / 215 1400
Shallow 193 / 197 1500
Remove subtree 129 / 166 1600
Queries on tree 96 / 145 1900
Queries on tree 2 68 / 75 1900
Library 45 / 55 1900
Black vertices 40 / 44 2000