1028.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 {
    public TreeNode recoverFromPreorder(String traversal) {
        Stack<TreeNode> stack = new Stack<>();
        int i = 0;

        while (i < traversal.length()) {
            int depth = 0;
            while (i < traversal.length() && traversal.charAt(i) == '-') {
                depth++;
                i++;
            }

            int num = 0;
            while (i < traversal.length() && Character.isDigit(traversal.charAt(i))) {
                num = num * 10 + (traversal.charAt(i) - '0');
                i++;
            }

            // Create the new node
            TreeNode newNode = new TreeNode(num);

            while (stack.size() > depth) {
                stack.pop();
            }
            if (!stack.isEmpty()) {
                if (stack.peek().left == null) {
                    stack.peek().left = newNode;
                } else {
                    stack.peek().right = newNode;
                }
            }

            stack.push(newNode);
        }
        return stack.isEmpty() ? null : stack.get(0);
    }
}