Update RecoverTreeFromPreorder.java

Author: vishrutimahajan

JAVA/RecoverTreeFromPreorder.java

@@ -0,0 +1,114 @@
+/*
+ * Problem: 1028. Recover a Tree From Preorder Traversal
+ * Link: https://leetcode.com/problems/recover-a-tree-from-preorder-traversal/
+ * 
+ * You are given a string traversal representing a preorder traversal of a binary tree.
+ * The string uses dashes '-' to indicate the depth of each node.
+ * Return the root of the tree that was traversed.
+ * 
+ * Example:
+ * Input: traversal = "1-2--3--4-5--6--7"
+ * Output: Preorder traversal of recovered tree → 1 2 3 4 5 6 7
+ * 
+ * Approach: Use a stack to maintain the correct parent node at each depth.
+ */
+
+
+
+/**
+ * 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;
+ *     }
+ * }
+ */
+import java.util.Stack;
+class TreeNode {
+    int val;
+    TreeNode left, right;
+    TreeNode(int x) {
+        val = x;
+        left = null;
+        right = null;
+    }
+}
+
+class Solution {
+    public TreeNode recoverFromPreorder(String traversal) {
+        Stack<TreeNode> stack = new Stack<>();
+        int i = 0, n = traversal.length();
+
+        while (i < n) {
+            int depth = 0;
+
+            // Count dashes to determine depth
+            while (i < n && traversal.charAt(i) == '-') {
+                depth++;
+                i++;
+            }
+
+            // Read the number (node value)
+            int numStart = i;
+            while (i < n && Character.isDigit(traversal.charAt(i))) {
+                i++;
+            }
+            int value = Integer.parseInt(traversal.substring(numStart, i));
+
+            TreeNode node = new TreeNode(value);
+
+            // If stack size > depth, pop until stack has correct depth parent
+            while (stack.size() > depth) {
+                stack.pop();
+            }
+
+            // Attach to parent if exists
+            if (!stack.isEmpty()) {
+                TreeNode parent = stack.peek();
+                if (parent.left == null) {
+                    parent.left = node;
+                } else {
+                    parent.right = node;
+                }
+            }
+
+            // Push current node to stack
+            stack.push(node);
+        }
+
+        // Root node is the first node in stack
+        while (stack.size() > 1) {
+            stack.pop();
+        }
+        return stack.peek();
+    }
+
+    // Helper function to print tree in preorder
+    public static void preorder(TreeNode root) {
+        if (root == null) return;
+        System.out.print(root.val + " ");
+        preorder(root.left);
+        preorder(root.right);
+    }
+
+    public static void main(String[] args) {
+        Solution sol = new Solution();
+        String traversal = "1-2--3--4-5--6--7";
+        TreeNode root = sol.recoverFromPreorder(traversal);
+
+        System.out.println("Preorder Traversal of Recovered Tree:");
+        preorder(root);
+    }
+
+    public boolean divideArray(int[] nums) {
+        // TODO Auto-generated method stub
+        throw new UnsupportedOperationException("Unimplemented method 'divideArray'");
+    }
+}