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 143 / 145 1200
Subtree update 139 / 142 1400
Ancestor 123 / 124 1400
Remove subtree 77 / 98 1500
Shallow 107 / 110 1500
Queries on tree 52 / 76 1900
Queries on tree 2 34 / 40 1900
Library 20 / 25 1900
Black vertices 15 / 19 2000