LeetCode-sum-root-to-leaf-numbers

Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path1->2->3which represents the number123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path1->2represents the number12.
The root-to-leaf path1->3represents the number13.

Return the sum = 12 + 13 =25.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        if(root==null)
            return 0;
         
        return helper(root,0);
    }
     
    private int helper(TreeNode root,int sum){
        if(root==null)
            return 0;
        sum=10*sum+root.val;
         
        if(root.left==null&&root.right==null)//////////必须加上，如果不加，root 处的sum 为0！！！
              return sum;
         
        int left=helper(root.left,sum);    //
        int right=helper(root.right,sum);
         
        return left+right;
    }
}