Smallest String Starting From Leaf

Given the root of a binary tree, each node has a value from 0 to 25 representing the letters 'a' to 'z': a value of 0 represents 'a', a value of 1 represents 'b', and so on.

Find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.

(As a reminder, any shorter prefix of a string is lexicographically smaller: for example, "ab" is lexicographically smaller than "aba".  A leaf of a node is a node that has no children.)

 


Example 1:

Input: [0,1,2,3,4,3,4]
Output: "dba"
Example 2:

Input: [25,1,3,1,3,0,2]
Output: "adz"
Example 3:

Input: [2,2,1,null,1,0,null,0]
Output: "abc"
 

Note:

  1. The number of nodes in the given tree will be between 1 and 8500.
  2. Each node in the tree will have a value between 0 and 25.

Solution:

/**
 * 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 {    
    String res = null;
    public String smallestFromLeaf(TreeNode root) {
        pre(root, new StringBuilder());
        return res;        
    }
    
    private void pre(TreeNode root, StringBuilder sb) {
        if (root == null) return;
        String s = sb.toString();
        sb.append((char) (root.val + 'a'));
        if (root.left == null && root.right == null) {
            String curr = sb.reverse().toString();
            if (res == null || curr.compareTo(res) < 0) {
                res = curr;
            } 
        }
        pre(root.left, sb);
        pre(root.right, sb);
        sb.setLength(0);
        sb.append(s);
    }
}