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 170 / 174 1200
Subtree update 162 / 165 1400
Ancestor 146 / 148 1400
Shallow 131 / 134 1500
Remove subtree 88 / 116 1600
Queries on tree 69 / 97 1900
Queries on tree 2 51 / 57 1900
Library 29 / 37 1900
Black vertices 27 / 30 2000