2096 - Step-By-Step Directions From a Binary Tree Node to Another

 JAVA

class Solution {
    static byte[] path = new byte[200_001];
    int strtLevel = -1; 
    int destLevel = -1;
    int comnLevel = -1;
    
    public String getDirections(TreeNode root, int startValue, int destValue) {
        findPaths(root, startValue, destValue, 100_000);
        int answerIdx = comnLevel;
        for (int i = strtLevel; i > comnLevel; i--)  
            path[--answerIdx] = 'U';
        return new String(path, answerIdx, destLevel - answerIdx);
    }
    
    private int findPaths(TreeNode node, int strtVal, int destVal, int level) {
        if (node == null)  return 0;
        int result = 0;
        if (node.val == strtVal) {
            strtLevel = level;
            result = 1;
        } else if (node.val == destVal) {
            destLevel = level;
            result = 1;
        }
        int leftFound = 0;
        int rightFound = 0;
        if (comnLevel < 0) {
            if (destLevel < 0)  path[level] = 'L';
            leftFound = findPaths(node.left, strtVal, destVal, level + 1);
            rightFound = 0;
            if (comnLevel < 0) {
                if (destLevel < 0)  path[level] = 'R';
                rightFound = findPaths(node.right, strtVal, destVal, level + 1);
            }
        }
        if (comnLevel < 0 && leftFound + rightFound + result == 2) 
            comnLevel = level;
        return result | leftFound | rightFound;
    }
}