leetcode-construct binary tree from inorder and postorder traversal

 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class  {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        if(inorder==null || postorder==null)
        	return null;
        //将元素和其下标存放在map中，便于寻找
        HashMap<Integer,Integer> map=new HashMap<Integer,Integer>();
        for(int i=0;i<inorder.length;i++)
        	map.put(inorder[i],i);
        return helper(inorder,0,inorder.length-1,postorder,0,postorder.length-1,map);
    }

    public TreeNode helper(int[] inorder,int inL,int inR,int[] postorder,int postL,int postR,HashMap<Integer,Integer> map)
    {
    	if(inL>inR || postL>postR)
    		return null;
    	TreeNode root=new TreeNode(postorder[postR]);
    	int val=root.val;
    	int index=map.get(val);
    	root.left=helper(inorder,inL,index-1,postorder,postL,postL+index-inL-1,map);
    	root.right=helper(inorder,index+1,inR,postorder,postR-inR+index,postR-1,map);
    	return root;
    }
}