129. Sum Root to Leaf Numbers.cpp

struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};
class Solution
{
public:
    int sumNumbers(TreeNode *root, int sum = 0)
    {
        if (!root)
            return 0;
        sum = sum * 10 + root->val;
        if (!root->left && !root->right)
            return sum;
        return sumNumbers(root->left, sum) + sumNumbers(root->right, sum);
    }
};