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.
解题思路：
递归，JAVA实现如下：
static public int sumNumbers(TreeNode root) {
if(root==null)
return 0;
return sumNumbers(root,0);
}
public static int sumNumbers(TreeNode root,int res) {
if(root.left==null&&root.right==null)
return res*10+root.val;
if(root.left==null)
return sumNumbers(root.right,res*10+root.val);
if(root.right==null)
return sumNumbers(root.left,res*10+root.val);
return sumNumbers(root.left,res*10+root.val)+sumNumbers(root.right,res*10+root.val);
}