Leetcode–Sum Root to Leaf Numbers

The Problem:

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.

Notes:

  1. This is a basic tree traversal problem. The key is to only add to final output at leaf nodes.

Java Solution

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int sum = 0;
    public int sumNumbers(TreeNode root) {
        if(root==null)
            return 0;
        sumPath(root, 0);
        return sum;
    }
    
    public void sumPath(TreeNode root, int pass){
        if(root.left==null && root.right==null){
            // leaf node, add to final result
            sum+= 10 * pass + root.val;
        }
        else {
            if(root.left!= null){
                 sumPath(root.left, 10 * pass + root.val);
            }
             if(root.right!= null){
                 sumPath(root.right, 10 * pass + root.val);
            }
        }
    }
}
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