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 299 / 304 1200
Subtree update 293 / 299 1400
Ancestor 257 / 259 1400
Shallow 237 / 241 1500
Remove subtree 151 / 195 1600
Queries on tree 108 / 170 1900
Queries on tree 2 77 / 91 1900
Library 51 / 63 1900
Tree rotation 13 / 14 1900
Black vertices 49 / 54 2000