106. Construct Binary Tree from Inorder and Postorder Traversal

class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        return helper(postorder.length-1, 0, inorder.length - 1, inorder, postorder);
        
    }
    
    public TreeNode helper(int post, int inStart, int inEnd, int[] inorder, int[] postorder) {
        if(inStart > inEnd || post < 0) {
            return null;
        }
        TreeNode root = new TreeNode(postorder[post]);
        int rootIndex = 0;
        for(int i = inStart; i <= inEnd; i++) {
            if(inorder[i] == root.val) {
                rootIndex = i;
                break;
            }
        }
        
        root.right = helper(post-1, rootIndex+1, inEnd, inorder, postorder);
        root.left = helper(post - (inEnd - rootIndex + 1), inStart, rootIndex - 1, inorder, postorder );
        // inEnd - rootIndex注意是inEnd去减， 不是inorder.length
        return root;
    }
}