brain storm - 1 year ago 117
Java Question

# construct binary search tree from Post-order traversal in Java

I am implementing code to construct

`BST(binary search tree)`
from a given
`post-order traversal array`
following this algorithm. I do not get back the
`binary Search Tree`
back. I am getting something that makes no-sense. here is my code

``````public class BinaryTreemethods {
public static void main(String[] args) {
int[] preOrder = { 5, 3, 1, 4, 8, 6, 9 };
int[] inOrder = { 1, 3, 4, 5, 6, 8, 9 };
int[] postOrder = {1,4,3,8,6,9,5};

static int postIndex=postOrder.length-1;
Node postordertree= buildBinarytreefromPostOrder(postOrder, 0, postOrder.length-1);
System.out.println("pre order traversal of node from postorder reconstructed tree ");
printPreOrder(postordertree);

}
private static void printPreOrder(Node tree) {
if (tree != null) {
System.out.print(" " + tree.data);
printPreOrder(tree.left);
printPreOrder(tree.right);
}

}
//this just reconstructs BT from post-order traversal elements
public static Node buildBinarytreefromPostOrder(int[] post, int start, int end){
if (postIndex<start || start > end ){
return null;
}

Node root = new Node(post[postIndex]);
postIndex--;
if (end == start){
//System.out.println("called");
return root;
}

int i = 0;
for (i=end;i>=start;i--){
if (post[i]<root.data)
break;
}

// Use the index of element found in postorder to divide postorder array
// in two parts. Left subtree and right subtree
root.right=buildBinarytreefromPostOrder(post,i+1, postIndex);
root.left=buildBinarytreefromPostOrder(post,start,i);
//root.left=buildBinarytreefromPostOrder(post,start,i);
//root.right=buildBinarytreefromPostOrder(post,i+1, postIndex);

return root;
}
}
``````

The output when I print in
`pre-order traversal is 5 9 6 8 3 4`
which is not correct.

Any idea where I could be going wrong?

EDIT: After swapping the order of lines for
`root.right and root.left`
(commented out one were before),
the
`left tree`
is build correctly, but the right tree is not. The output I get is
`5 3 1 4 9 6 8`

As the root of each subtree you are taking `postIndex` which is global for the whole structure. You should take the last element of the subarray (`end`).

It should rather be like this

``````public static Node buildBinarytreefromPostOrder(int[] post, int start, int end)
{
if (end < start)
return null;

Node root = new Node(post[end]);

if (end == start)
return root;

int i;
for (i = end; i >= start; i--)
if (post[i] < root.data)
break;

root.left = buildBinarytreefromPostOrder(post, start, i);
root.right = buildBinarytreefromPostOrder(post, i + 1, end - 1);

return root;
}
``````
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