二叉树重建-递归

package JianZhioffer;
// 输入某二叉树的前序遍历和中序遍历的结果，请重建该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。
//递归，进入helperZ之前用整体进入后分左右
public class test07 {
    public static void main(String[] args) {
       int []preorder={3,9,20,15,7};
       int []inorder={9,3,15,20,7};
       TreeNode tr=buildTree(preorder, inorder);
       int x=0;
    }
    public static TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder.length==0||inorder.length==0){
            return null;
        }
        
        TreeNode result=helper(0, preorder.length-1, 0,inorder.length-1,preorder, inorder);
        return result;
    }
    public static TreeNode helper(int prestart,int preend,int instart,int inend,int []preorder,int []inorder){
        
        if( prestart>preend){
            return null;
        }
        TreeNode tr=new TreeNode(preorder[prestart]);
        if(prestart==preend){
            return tr;
        }
        
        int r=preorder[prestart];
        int k=0;
        for(int i=instart;i<=inend;i++){
            if(inorder[i]==r){
                k=i;
                break;
            }
        }
        int x=k-instart;
        int y=inend-k;
        tr.left=helper(prestart+1, prestart+x,instart,k-1, preorder, inorder);
        tr.right=helper(preend-y+1, preend,k+1,inend, preorder, inorder);

        return tr;
    }
}