Day-27 Construct Binary Tree from Inorder and Postorder Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    HashMap<Integer,Integer> map=null;
    int post_index=0;
    
    public TreeNode buildTree(int[] inorder, int[] post) {
        map=new HashMap<>();
        post_index=post.length-1;
        
        for(int i=0;i<inorder.length;i++){
            map.put(inorder[i],i);
        }
        return construct(post,0,inorder.length-1);
    }
    
    public TreeNode construct(int[] post,int instart,int inend){
        if(instart>inend) return null;
        
        int index=map.get(post[post_index]);
                
        TreeNode root=new TreeNode(post[post_index]);
        
        post_index--;
        
        root.right=construct(post,index+1,inend);
        root.left=construct(post,instart,index-1);
         
        return root;
    }
}