LeetCode2096: Step-By-Step Directions From a Binary Tree Node to Another

Algorithm: Find the Strings from root to start and root to dest. And then remove the common prefix of both the strings. Replace each character of start string with ‘U’ . And concatinate the start string with the reversed dest string.

/**
 * 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 {
    StringBuilder path = new StringBuilder("");
    public String getDirections(TreeNode root, int startValue, int destValue) {
        getStringfromRootToNode(root, startValue);
        StringBuilder start = path.reverse();
        path=new StringBuilder("");
        getStringfromRootToNode(root, destValue);
        StringBuilder dest= path.reverse();
        removeCommonPrefix(start,dest);
        replaceStartString(start);
        return concatinateStrings(start,dest);
        
    }
    
    public Boolean getStringfromRootToNode(TreeNode root , int value){
        // return reverse strings from root to valueNode
        if(root.val==value) return true;
        if(root.left!=null && getStringfromRootToNode(root.left,value)) {
            path.append("L");
           return true; 
        }
        if(root.right!=null && getStringfromRootToNode(root.right,value)) {
            path.append("R");
            return true;
        }
        return false;
          
    }
    
    public void removeCommonPrefix(StringBuilder s1, StringBuilder s2){
        //remove common prefix 
        while(s1.length()!=0 && s2.length()!=0
             && s1.charAt(0)==s2.charAt(0)){
            s1.deleteCharAt(0);
            s2.deleteCharAt(0);
            
        }
    }
    
    public void replaceStartString( StringBuilder s){
        int i=0;
        while(i<s.length()){
            s.setCharAt(i,'U');
                i++;
        }
    }
    
    
    public String concatinateStrings(StringBuilder s1, StringBuilder s2){
        //concatinate s1 with Us and reversed s2
        s1.append(s2);
        return s1.toString();
    }
    
}