README.md

backtracking

1. We can use backtracking to go through all the path from root to each of the leaf nodes.
   
2. What we need to pass down:
   • need to know the contribution of cur node (10^level)
   • cur path value time: O(n) - n is the number of nodes
     space: O(height)

pure recursion

For each node, we return: <sum of all paths, # of leaf nodes>0 time: O(n) - n is the number of nodes
space: O(height)