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.

题目含义：根节点到叶子节点的元素依次从高到地位构成一个整数，求所有整数的和

    private int sums(TreeNode root,int sum) {
        if (root ==null) return 0;
        sum = sum*10 + root.val;
        if (root.left == null && root.right==null) return sum;
        return  sums(root.left,sum) + sums(root.right,sum);
    }
    
    public int sumNumbers(TreeNode root) {
        return sums(root,0);
    }