[GeeksForGeeks] Get postorder traversal sequence from given inorder and preorder traversal.

Given Inorder and Preorder traversals of a binary tree, print Postorder traversal.

Example:

Input:
Inorder traversal in[] = {4, 2, 5, 1, 3, 6}
Preorder traversal pre[] = {1, 2, 4, 5, 3, 6}

Output:
Postorder traversal is {4, 5, 2, 6, 3, 1}

 

A naive solution is to first construct the given tree, then recursively traverse it in post order.

 

A better solution is to acheive this without having to construct the given tree first.

 

import java.util.ArrayList;

public class getPostorder {
    public ArrayList<Integer> getPostorderSeq(int[] in, int[] pre) {
        if(in == null || pre == null || in.length != pre.length || in.length == 0) {
            return null;
        }
        ArrayList<Integer> post = new ArrayList<Integer>();        
        helper(in, 0, in.length - 1, pre, 0, pre.length - 1, post);
        return post;
    }
    private void helper(int[] in, int in_start, int in_end,
                        int[] pre, int pre_start, int pre_end,
                        ArrayList<Integer> post) {
        if(in_start > in_end) {
            return;
        }
        int rootIdx = pre_start;
        int inOrderRootIdx = in_start;
        for(; inOrderRootIdx <= in_end; inOrderRootIdx++) {
            if(pre[rootIdx] == in[inOrderRootIdx]) {
                break;
            }
        }
        helper(in, in_start, inOrderRootIdx - 1, pre,  pre_start + 1, pre_start + inOrderRootIdx - in_start, post);
        helper(in, inOrderRootIdx + 1, in_end, pre, pre_end - in_end + inOrderRootIdx + 1, pre_end, post);
        post.add(pre[rootIdx]);
    }
}