129. Sum Root to Leaf Numbers

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.

求和，二叉树的遍历，dfs求解即可，程序如下所示：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int currentVal = 0, sum = 0;
    public int sumNumbers(TreeNode root) {
        if (root == null){
            return 0;
        }
        dfs(root);
        return sum;
    }
    
    public void dfs(TreeNode root){
        if (root == null){
            return;
        }
        currentVal = currentVal*10 + root.val;
        if (root.left == null&&root.right == null){
            sum += currentVal;
        }
        dfs(root.left);
        dfs(root.right);
        currentVal = (currentVal - root.val)/10;
    }
}