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:
    string getDirections(TreeNode* root, int startValue, int destValue) {
        TreeNode* node = lca(root, startValue, destValue);
        string pathToStart, pathToDest;
        dfs(node, startValue, pathToStart);
        dfs(node, destValue, pathToDest);
        return string(pathToStart.size(), 'U') + pathToDest;
    }

private:
    TreeNode* lca(TreeNode* node, int p, int q) {
        if (node == nullptr || node->val == p || node->val == q) {
            return node;
        }
        TreeNode* left = lca(node->left, p, q);
        TreeNode* right = lca(node->right, p, q);
        if (left != nullptr && right != nullptr) {
            return node;
        }
        return left != nullptr ? left : right;
    }

    bool dfs(TreeNode* node, int x, string& path) {
        if (node == nullptr) {
            return false;
        }
        if (node->val == x) {
            return true;
        }
        path.push_back('L');
        if (dfs(node->left, x, path)) {
            return true;
        }
        path.back() = 'R';
        if (dfs(node->right, x, path)) {
            return true;
        }
        path.pop_back();
        return false;
    }
};