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.
// #[derive(Debug, PartialEq, Eq)]
// pub struct TreeNode {
//   pub val: i32,
//   pub left: Option<Rc<RefCell<TreeNode>>>,
//   pub right: Option<Rc<RefCell<TreeNode>>>,
// }
//
// impl TreeNode {
//   #[inline]
//   pub fn new(val: i32) -> Self {
//     TreeNode {
//       val,
//       left: None,
//       right: None
//     }
//   }
// }
use std::cell::RefCell;
use std::rc::Rc;
impl Solution {
    fn dfs(node: &Option<Rc<RefCell<TreeNode>>>, mut num: i32) -> i32 {
        if node.is_none() {
            return 0;
        }
        let node = node.as_ref().unwrap().borrow();
        num = num * 10 + node.val;
        if node.left.is_none() && node.right.is_none() {
            return num;
        }
        Self::dfs(&node.left, num) + Self::dfs(&node.right, num)
    }

    pub fn sum_numbers(root: Option<Rc<RefCell<TreeNode>>>) -> i32 {
        Self::dfs(&root, 0)
    }
}
