Insufficient Nodes in Root to Leaf Paths — C++

// Time:  O(n)
// Space: O(h)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* sufficientSubset(TreeNode* root, int limit) {
        if (!root) {
            return nullptr;
        }
        if (!root->left && !root->right) {
            return root->val < limit ? nullptr : root;
        }
        root->left = sufficientSubset(root->left, limit - root->val);
        root->right = sufficientSubset(root->right, limit - root->val);
        if (!root->left && !root->right) {
            return nullptr;
        }
        return root;
    }
};