find max. leaf nodes of a binary tree

       50
     /     
   25      75
  /       / 
 15  27   62  79  

depth = 3
total nodes = 7
max leaf nodes = 4

       50
     /     
   25      75
  /       / 
 15  27   62  79  
 _    _   _    _ 
 1    1   1    1  = 4

in a case where the Tree is full this will find the total amount of leaf nodes, but if the tree is not full and only the depth of the tree is known how can i know the max. leaf nodes in that tree?

// if the tree is full
function total_leafs_nodes(node, level, mxlevel)
  {
      if (node == null) {
        return 1;
      }
      if (node.left == null && node.right == null)
          return 1;

      else if (level == mxlevel)
      {
          return 1;
      }
      return (total_leafs_nodes(node.left, level + 1, mxlevel)
      + total_leafs_nodes(node.right, level + 1, mxlevel));
  }