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 147 / 149 1200
Subtree update 143 / 146 1400
Ancestor 125 / 126 1400
Remove subtree 78 / 100 1500
Shallow 111 / 114 1500
Queries on tree 56 / 81 1900
Queries on tree 2 37 / 44 1900
Library 22 / 28 1900
Black vertices 15 / 19 2000