Sum Root to Leaf Numbers

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
const int mod = 1003;
void helper(TreeNode* A, int& res, long long now) {
    if(!A) return;
    now = (now * 10 + A->val) % mod;
    if(!A->left and !A->right) res = (res + now) % mod;
    else {
        helper(A->left,res,now);
        helper(A->right,res,now);
    }
}
int Solution::sumNumbers(TreeNode* A) {
    int res = 0;
    helper(A,res,0);
    return res;
}