[LeetCode]Sum Root to Leaf Numbers

Question
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.

---

本题难度Medium。

DFS

【复杂度】
时间 O(N) 空间 O(1)

【思路】
这道题并不难，从上而下地进行递推累加，然后返回其累加值。

【注意】
叶子节点是其左右子树都为null。只有一个子树为null的不是叶子节点！

【代码】

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        //invariant
        return helper(root,0);
    }
    private int helper(TreeNode root,int pre){
        //base case
        if(root==null)
            return 0;
        if(root.left==null&&root.right==null)
            return pre*10+root.val;

        return helper(root.left,pre*10+root.val)+helper(root.right,pre*10+root.val);
    }
}