[Leetcode] 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.

比较简单的树的遍历问题，主要就是找到叶子节点然后返回值。

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