View Compilation Error

0_0_13799533_16133.cpp:13:14: error: 'null_mapped_type' was not declared in this scope
     tree<int,null_mapped_type,less<int>,rb_tree_tag,tree_order_statistics_node_update>s;
              ^
0_0_13799533_16133.cpp:13:86: error: template argument 2 is invalid
     tree<int,null_mapped_type,less<int>,rb_tree_tag,tree_order_statistics_node_update>s;
                                                                                      ^