计算树的最大深度

  
 private int CalMaxLevel( TreeNode currentNode )
{
       if( currentNode == null )
      {
            return -1;
      }
       int level = 1;
       int maxLevel = 0;
       foreach( TreeNode node in currentNode.Childrens )
      {
            level = CalMaxLevel( node ) + 1 ;
            if( maxLevel < level)
            {
               maxLevel = level;
            }
      }   
      return maxLevel;   
}