Sum Root to Leaf Numbers | Leetcode

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.

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Solution: Recurse down the tree, but don't recurse onto null nodes. For each recursion, multiply the previous value by 10 and add the current node's value. When you reach a leaf node, return the value. If you hit a non-leaf node, take the sum of the left and right children, but count null children nodes as zero.

/**
 * Definition for a binary tree node.
 * 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;
        return sum(root, 0);
    }
    
    private int sum(TreeNode node, int n) {
        int i = (n * 10) + node.val;
        if (node.left == null && node.right == null) return i;
        
        int s = 0;
        if (node.left != null) s += sum(node.left, i);
        if (node.right != null) s += sum(node.right, i);
        return s;
    }
}