input:
                    1                  
				 /      \
			   2          3           
			 /  \           \
		  4      5           6       
		 /         \           \
	   7            8           9
	   
output:
	                 1    -- null              
				 /      \
			   2    --    3    -- null       
			 /  \           \
		  4  --  5    ---    6    -- null    
		 /          \          \
	   7    ----      8  ---     9  --- null 

Explaination:
root1.s=null
root2.s=root3
root3.s=null 
root4.s=root5 continue..
	   
  class NodeBT {
    
	int val;
	NodeBT left;
    NodeBT right;
    NodeBT s;
  
    public NodeBT(int val) {
	   left=null;
	   right=null;
	   s=null;
	   this.val=val;
	      }
	 }