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 106 / 108 1200
Subtree update 102 / 104 1400
Ancestor 87 / 88 1400
Remove subtree 53 / 72 1500
Shallow 70 / 73 1500
Queries on tree 34 / 49 1900
Queries on tree 2 22 / 26 1900
Library 6 / 10 1900
Black vertices 9 / 11 2000