Leetcode -- 129. Sum Root to Leaf Numbers(dfs)

129. Sum Root to Leaf Numbers(dfs)
medium
https://leetcode.com/problems/sum-root-to-leaf-numbers/#/description

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.

思路： - Using dfs, add left path & right path to current node for getting number till reach the leaf.
           Calculate by sum = root.val + sum*10;
           - Since we calculate left & right with same previous sum(parent node), so we do
            recursive call: helper(root.left, sum) + helper(root.right, sum);

Complexity： Time O(n) Space O(h)

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