Nested Loop * unique tree problem (i've searched already)

[psherman]

I need help figuring out how to design a code to output:

     *
    ***
     *
    ***
   *****
     *
    ***
   *****
  ******* 

...and so on based on two input values. I've got the general concept down i think but am having trouble making the triangle repeat.

  #include<iostream>
using namespace std;
int main()
{
        int rows = 0;
        int lines;

        cout<< "Number of lines: "<< endl;
        cin>> lines;

        for(int i=0; i< lines; i++)
        {
                for(int j = (lines -1 -i); j> 0; j--)
                {
                        cout<<" ";
                }
                for(int k=0; k < (i*2 +1); k++)
                {
                        cout<< "*";
                }

                cout<<endl;
        }
        return 0;
}

When I output this it gives me:

  
   *
  ***
 *****
*******

...based on the two values i put in, so I know I'm close. I just need a little help to finish.