Sum root to leaf

/**
 * SumRootToLeafNumbers.java
 * 
 * Description: Given a binary tree containing digits from 0-9 only,
each
 * root-to-leaf path could represent a number.
 * 
 * An example is the root-to-leaf path 1->2->3 which represents the
number 123.
 * 
 * Find the total sum of all root-to-leaf numbers.
 * 
 * For example,
 * 
 * 		1 
 *     / \ 
 *    2   3 
 * 
 * The root-to-leaf path 1->2 represents the number 12. 
 * The root-to-leaf path 1->3 represents the number 13.
 * 
 * Return the sum = 12 + 13 = 25.
 * 
 * @author John Qiao <qiaoshun8888@gmail.com> Date: Jan 29, 2014
 */