Document difference between depth-first search for graphs O(v + e) and for binary trees O(v)

[enriquedlh97]

The problem branch sums which uses depth first search for a binary tree has a time complexity of O(v) where v are the vertices in the tree. Using the same DFS strategy in a graph in the problem depth-first search, the time complexity is O(v + e) where v once again corresponds to the vertices in the graph and e to the edges.

Explain in the documentation fo the solution why the same approach yields different time complexities.