Checking pathSum from root to leaf

Here in this method, we traverse from the ROOT right upto the LEAF.
 Given a tree and a sum, returns true if there is a path from the root 
 down to a leaf, such that adding up all the values along the path 
 equals the given sum.
 Strategy: subtract the node value from the sum when recurring down, 
 and check to see if the sum is 0 when you run out of tree.