1305. All Elements in Two Binary Search Trees

Given two binary search trees root1 and root2, return a list containing all the integers from both trees sorted in ascending order.

/**
 * Definition for a binary tree node.
 * 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 {
    void helper(vector<int>& res, vector<TreeNode*>& st) {
        res.push_back(st.back()->val);
        auto node = st.back(); st.pop_back();
        if(node->right) {
            st.push_back(node->right);
            node = node->right->left;
            while(node) {
                st.push_back(node);
                node = node->left;
            }
        }
    }
public:
    vector<int> getAllElements(TreeNode* root1, TreeNode* root2) {
        vector<TreeNode*> st1, st2;
        vector<int> res;
        while(root1) {
            st1.push_back(root1);
            root1 = root1->left;
        }
        while(root2) {
            st2.push_back(root2);
            root2 = root2->left;
        }
        while(!st1.empty() and !st2.empty()) {
            auto& st = st1.back()->val < st2.back()->val ? st1 : st2;
            helper(res, st);
        }
        while(!st1.empty()) {
            helper(res, st1);
        }
        while(!st2.empty()) {
            helper(res, st2);
        }
        return res;
    }
};