Sum Root to Leaf Numbers
<p>You are given the <code>root</code> of a binary tree containing digits from <code>0</code> to <code>9</code> only.</p>

<p>Each root-to-leaf path in the tree represents a number.</p>

<ul>
	<li>For example, the root-to-leaf path <code>1 -&gt; 2 -&gt; 3</code> represents the number <code>123</code>.</li>
</ul>

<p>Return <em>the total sum of all root-to-leaf numbers</em>. Test cases are generated so that the answer will fit in a <strong>32-bit</strong> integer.</p>

<p>A <strong>leaf</strong> node is a node with no children.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0129.Sum%20Root%20to%20Leaf%20Numbers/images/num1tree.jpg" style="width: 212px; height: 182px;" />
<pre>
<strong>Input:</strong> root = [1,2,3]
<strong>Output:</strong> 25
<strong>Explanation:</strong>
The root-to-leaf path <code>1-&gt;2</code> represents the number <code>12</code>.
The root-to-leaf path <code>1-&gt;3</code> represents the number <code>13</code>.
Therefore, sum = 12 + 13 = <code>25</code>.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0129.Sum%20Root%20to%20Leaf%20Numbers/images/num2tree.jpg" style="width: 292px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [4,9,0,5,1]
<strong>Output:</strong> 1026
<strong>Explanation:</strong>
The root-to-leaf path <code>4-&gt;9-&gt;5</code> represents the number 495.
The root-to-leaf path <code>4-&gt;9-&gt;1</code> represents the number 491.
The root-to-leaf path <code>4-&gt;0</code> represents the number 40.
Therefore, sum = 495 + 491 + 40 = <code>1026</code>.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 1000]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 9</code></li>
	<li>The depth of the tree will not exceed <code>10</code>.</li>
</ul>
Tree; Depth-First Search; Binary Tree
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int sumNumbers(TreeNode root) {
        return dfs(root, 0);
    }

    private int dfs(TreeNode root, int s) {
        if (root == null) {
            return 0;
        }
        s = s * 10 + root.val;
        if (root.left == null && root.right == null) {
            return s;
        }
        return dfs(root.left, s) + dfs(root.right, s);
    }
}