Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private Map<Integer, List<List<String>>> edges;
    private Set<Integer> visited;
    private String ans;

    public String getDirections(TreeNode root, int startValue, int destValue) {
        edges = new HashMap<>();
        visited = new HashSet<>();
        ans = null;
        traverse(root);
        dfs(startValue, destValue, new ArrayList<>());
        return ans;
    }

    private void traverse(TreeNode root) {
        if (root == null) {
            return;
        }
        if (root.left != null) {
            edges.computeIfAbsent(root.val, k -> new ArrayList<>())
                .add(Arrays.asList(String.valueOf(root.left.val), "L"));
            edges.computeIfAbsent(root.left.val, k -> new ArrayList<>())
                .add(Arrays.asList(String.valueOf(root.val), "U"));
        }
        if (root.right != null) {
            edges.computeIfAbsent(root.val, k -> new ArrayList<>())
                .add(Arrays.asList(String.valueOf(root.right.val), "R"));
            edges.computeIfAbsent(root.right.val, k -> new ArrayList<>())
                .add(Arrays.asList(String.valueOf(root.val), "U"));
        }
        traverse(root.left);
        traverse(root.right);
    }

    private void dfs(int start, int dest, List<String> t) {
        if (visited.contains(start)) {
            return;
        }
        if (start == dest) {
            if (ans == null || ans.length() > t.size()) {
                ans = String.join("", t);
            }
            return;
        }
        visited.add(start);
        if (edges.containsKey(start)) {
            for (List<String> item : edges.get(start)) {
                t.add(item.get(1));
                dfs(Integer.parseInt(item.get(0)), dest, t);
                t.remove(t.size() - 1);
            }
        }
    }
}