129. Sum Root to Leaf Numbers

Problem:

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.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2:

Input: [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

7/5/2018 update
第二遍也是想的brute force。但面试的时候有可能在面试官提示下做出高级方法。。。
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Analysis:

略

Solution:

class Solution {
    public int sumNumbers(TreeNode root) {
        return helper(root, 0);
    }

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