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

.

This is an easy problem: recursion without tracing back

/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int sumNumbers(TreeNode root) {
if(root == null) return 0;

int[] ans = new int[1];
sumRec(root,0, ans);
return ans[0];
}

private void sumRec(TreeNode root, int prev, int[] ans){
if(root.left == null && root.right == null){
ans[0] += (prev*10 + root.val);
return;
}

if(root.left != null) sumRec(root.left, prev*10+root.val, ans);
if(root.right != null) sumRec(root.right, prev*10+root.val, ans);
}
}

Time complexity is O(n）,  where n is the number of the nodes, each visited exactly once.

Space Complexity is O(n), too