View Compilation Error

0_0_30193847_2883.cpp:20:26: error: no matching function for call to 'node::node()'
 }a[maxn][maxn*maxn/2+maxn];
                          ^
0_0_30193847_2883.cpp:20:26: note: candidates are:
0_0_30193847_2883.cpp:19:5: note: node::node(ll, ll)
     node(ll _ac,ll _ak){ac=_ac,ak=_ak;}
     ^
0_0_30193847_2883.cpp:19:5: note:   candidate expects 2 arguments, 0 provided
0_0_30193847_2883.cpp:16:8: note: constexpr node::node(const node&)
 struct node{
        ^
0_0_30193847_2883.cpp:16:8: note:   candidate expects 1 argument, 0 provided
0_0_30193847_2883.cpp:16:8: note: constexpr node::node(node&&)
0_0_30193847_2883.cpp:16:8: note:   candidate expects 1 argument, 0 provided