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 203 / 206 1200
Subtree update 199 / 202 1400
Ancestor 174 / 176 1400
Shallow 159 / 161 1500
Remove subtree 103 / 137 1600
Queries on tree 80 / 114 1900
Queries on tree 2 58 / 65 1900
Library 35 / 45 1900
Black vertices 34 / 37 2000