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