Spoj Java Question on Tree

TREEORD.java

@@ -0,0 +1,94 @@
+// https://www.spoj.com/problems/TREEORD/
+// Accepted
+
+import java.io.*;
+import java.lang.*;
+import java.util.*;
+
+class TreeNode
+{
+    int id;
+    TreeNode left, right;
+
+    public TreeNode(int id) {
+        this.id = id;
+        left = right = null;
+    }
+}
+
+public class TREEORD
+{
+    public static int index;
+    public static boolean valid;
+
+    public static void getInput(int[] order, Scanner scan) {
+        for(int i = 0; i < order.length; i++) {
+            order[i] = scan.nextInt();
+        }
+    }
+
+    public static void postOrderTraversal(TreeNode root, int[] postorder)
+    {
+        if(root == null) return;
+
+        if(valid) postOrderTraversal(root.left, postorder);
+        if(valid) postOrderTraversal(root.right,postorder);
+
+        if(root.id != postorder[index++]) {
+            valid = false;
+        }
+    }
+
+    public static TreeNode constructTree(int start, int end,
+    int[] preorder, HashMap<Integer,Integer> map)
+    {
+        if(start > end) return null;
+
+        int key = preorder[index++];
+        TreeNode head = new TreeNode(key);
+
+        head.left = constructTree(start, map.get(key)-1, preorder, map);
+        head.right = constructTree(map.get(key)+1, end, preorder, map);
+
+        return head;
+    }
+
+    public static void main(String[] args) {
+
+        Scanner scan = new Scanner(System.in);
+
+        int n = scan.nextInt();
+
+        int pre[] = new int[n];
+        int pos[] = new int[n];
+        int ino[] = new int[n];
+
+        getInput(pre, scan);
+        getInput(pos, scan);
+        getInput(ino, scan);
+
+        HashMap<Integer, Integer> map = new HashMap<>();
+        for(int i = 0; i < n; i++) {
+            map.put(ino[i], i);
+        }
+
+        index = 0;
+        TreeNode root = constructTree(0, n-1, pre, map);
+
+        index = 0;
+        valid = true;
+        postOrderTraversal(root, pos);
+
+        if(valid == true) {
+            System.out.println("yes");
+        }
+        else {
+            System.out.println("no");
+        }
+    }
+}
+
+// 1 2 4 5 3 6 | pre
+// 4 2 5 1 3 6 | inorder
+
+// 4 5 2 6 3 1 | post