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 180 / 183 1200
Subtree update 170 / 173 1400
Ancestor 155 / 157 1400
Shallow 144 / 146 1500
Remove subtree 94 / 124 1600
Queries on tree 74 / 105 1900
Queries on tree 2 53 / 61 1900
Library 30 / 39 1900
Black vertices 30 / 33 2000