利用中序与后序遍历结果创建树

利用中序与后序遍历结果创建树

class TreeNode {
     int val;
    TreeNode left;
      TreeNode right;
  TreeNode(int x) { val = x; }
  }

  //利用中序与后序遍历结果创建树
public class Test{
      int[] inorder;
      int[] postorder;
      int post_index;
    Map<Integer,Integer> map =new HashMap<Integer, Integer>();
    public TreeNode buildTree(int[] inorder,int[] postorder)
    {
        post_index=postorder.length-1;
        for(int i=0;i<inorder.length;i++)
        {
            map.put(inorder[i],i);
        }
        this.inorder=inorder;
        this.postorder=postorder;
        return helper(0,inorder.length-1);
    }
    public TreeNode helper(int in_left,int in_right)
    {
        if(in_left>in_right)
            return  null;
        int root_val = postorder[post_index--];
        Integer index = map.get(root_val);
        TreeNode root = new TreeNode(root_val);
        root.right = helper(index+1,in_right);
        root.left = helper(in_left,index-1);
        return  root;
    }