Merge pull request #438 from prtk418/prtk418_binary_tree_construction_from_inorder_and_postorder_in_java

construct BST from inorder and postorder in java

Author: t2013anurag

Binary_Trees/constructBST_Using_Inorder_Postorder.java

@@ -0,0 +1,97 @@
+class Node 
+{
+    int data;
+    Node left, right;
+ 
+    public Node(int data) 
+    {
+        this.data = data;
+        left = right = null;
+    }
+}
+ 
+// Class Index created to implement pass by reference of Index
+class Index 
+{
+    int index;
+}
+ 
+class BinaryTree 
+{
+    /* Recursive function to construct binary of size n
+       from  Inorder traversal in[] and Preorder traversal
+       post[].  Initial values of inStrt and inEnd should
+       be 0 and n -1.  The function doesn't do any error
+       checking for cases where inorder and postorder
+       do not form a tree */
+    Node buildUtil(int in[], int post[], int inStrt,
+            int inEnd, Index pIndex) 
+    {
+        // Base case
+        if (inStrt > inEnd)
+            return null;
+ 
+        /* Pick current node from Preorder traversal using
+           postIndex and decrement postIndex */
+        Node node = new Node(post[pIndex.index]);
+        (pIndex.index)--;
+ 
+        /* If this node has no children then return */
+        if (inStrt == inEnd)
+            return node;
+ 
+        /* Else find the index of this node in Inorder
+           traversal */
+        int iIndex = search(in, inStrt, inEnd, node.data);
+ 
+        /* Using index in Inorder traversal, construct left and
+           right subtress */
+        node.right = buildUtil(in, post, iIndex + 1, inEnd, pIndex);
+        node.left = buildUtil(in, post, inStrt, iIndex - 1, pIndex);
+ 
+        return node;
+    }
+ 
+    // This function mainly initializes index of root
+    // and calls buildUtil()
+    Node buildTree(int in[], int post[], int n) 
+    {
+        Index pIndex = new Index();
+        pIndex.index = n - 1;
+        return buildUtil(in, post, 0, n - 1, pIndex);
+    }
+ 
+    /* Function to find index of value in arr[start...end]
+       The function assumes that value is postsent in in[] */
+    int search(int arr[], int strt, int end, int value) 
+    {
+        int i;
+        for (i = strt; i <= end; i++) 
+        {
+            if (arr[i] == value)
+                break;
+        }
+        return i;
+    }
+ 
+    /* This funtcion is here just to test  */
+    void preOrder(Node node) 
+    {
+        if (node == null)
+            return;
+        System.out.print(node.data + " ");
+        preOrder(node.left);
+        preOrder(node.right);
+    }
+ 
+    public static void main(String[] args) 
+    {
+        BinaryTree tree = new BinaryTree();
+        int in[] = new int[]{4, 8, 2, 5, 1, 6, 3, 7};
+        int post[] = new int[]{8, 4, 5, 2, 6, 7, 3, 1};
+        int n = in.length;
+        Node root = tree.buildTree(in, post, n);
+        System.out.println("Preorder of the constructed tree : ");
+        tree.preOrder(root);
+    }
+}