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 194 / 197 1200
Subtree update 188 / 191 1400
Ancestor 165 / 167 1400
Shallow 152 / 154 1500
Remove subtree 102 / 132 1600
Queries on tree 79 / 110 1900
Queries on tree 2 58 / 65 1900
Library 33 / 43 1900
Black vertices 33 / 36 2000