Solution.cpp

/**
 * 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 {
public:
    bool twoSumBSTs(TreeNode* root1, TreeNode* root2, int target) {
        vector<int> nums[2];
        function<void(TreeNode*, int)> dfs = [&](TreeNode* root, int i) {
            if (!root) {
                return;
            }
            dfs(root->left, i);
            nums[i].push_back(root->val);
            dfs(root->right, i);
        };
        dfs(root1, 0);
        dfs(root2, 1);
        int i = 0, j = nums[1].size() - 1;
        while (i < nums[0].size() && j >= 0) {
            int x = nums[0][i] + nums[1][j];
            if (x == target) {
                return true;
            }
            if (x < target) {
                ++i;
            } else {
                --j;
            }
        }
        return false;
    }
};