The edge list format is very simple: every tree is represented with a list of all its edges with indices from 0 to n-1 (where n denotes the number of vertices). The list of edges ends at the end of the line.
In the head vector format, every tree is represented with a vector of parent vertices (each consisting of a single line): the integer at position p in the list denotes the parent’s index of the p-th vertex, and a 0 denotes the only vertex with no parent. Example 1: the head vector “0 1 1 1 1” represents a star tree where the hub is vertex 0. Example 2: the head vector “0 1 2 3 4” represents a linear tree where the root vertex is vertex 0.