Find Largest Subtree Sum

Given a binary tree, task is to find subtree with maximum sum in tree.

Input :       1
            /   \
           2      3
          / \    / \
         4   5  6   7
Output : 28
As all the tree elements are positive,
the largest subtree sum is equal to
sum of all tree elements.

Input :       1
            /    \
          -2      3
          / \    /  \
         4   5  -6   2
Output : 7
Subtree with largest sum is :  -2
                             /  \ 
                            4    5
Also, entire tree sum is also 7.