Tree - (Recursion|Induction) Algorithm
1 - About
A recursive algorithms invoke themselves as a subroutine with a smaller input.
The idea of the recursion tree method is to write out all of the work done by the recursive algorithm in a tree structure, with the children of a given node corresponding to the recursive calls made by that node.
2 - Articles Related
3 - Base case
Recursive algorithms need a base case so they don't keep calling themselves until the rest of time. When the input's sufficient (small), you stop the recursion and you just return some trivial answer.