129. Sum Root to Leaf Numbers

Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path1->2->3which represents the number123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path1->2represents the number12.
The root-to-leaf path1->3represents the number13.

Return the sum = 12 + 13 =25.

Analysis:

Regular tree recursive traverse, when reach a null node, we add the sum of that path to final result.

Complexity:

Time: O(N)

Space: O(logn)

Code:

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

    private void sum(TreeNode root, int s)
    {
        if(root == null)
        {
            return;
        }
        if(root.left == null && root.right == null)
        {
            returnSum += root.val + s*10;
            return;
        }
        int tempSum = s*10 + root.val;
        sum(root.left, tempSum);
        sum(root.right, tempSum);
    }
}