Add TreeProblems class

Author: manojbaishya

src/main/java/org/dojo/algorithms/TreeProblems.java

@@ -1,4 +1,21 @@
 package org.dojo.algorithms;
 
+import java.util.HashMap;
+import java.util.Map;
+
 public class TreeProblems {
+    public static void processPostOrder(int[] inorder, int[] preorder) {
+        Map<Integer, Integer> inorderMap = new HashMap<>();
+        for (int i = 0; i < inorder.length; i++) inorderMap.put(inorder[i], i);
+        processPostOrderInternal(inorderMap, preorder, 0, preorder.length);
+    }
+
+    private static void processPostOrderInternal(Map<Integer, Integer> inorderMap, int[] preorder, int start, int end) {
+        if (start > end) return;
+        int node = inorderMap.get(preorder[0]);
+        if (node != 0) processPostOrderInternal(inorderMap, preorder, start, node - 1);
+        if (node != preorder.length - 1) processPostOrderInternal(inorderMap, preorder, node + 1, end);
+
+        System.out.println("Node is " + preorder[0]);
+    }
 }