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 210 / 213 1200
Subtree update 205 / 208 1400
Ancestor 180 / 182 1400
Shallow 163 / 168 1500
Remove subtree 107 / 141 1600
Queries on tree 80 / 115 1900
Queries on tree 2 58 / 65 1900
Library 36 / 46 1900
Black vertices 36 / 40 2000