View Compilation Error

0_0_27686067_765.cpp:5:25: error: array bound is not an integer constant before ']' token
 int a[50+5], pos[maxn<<1];
                         ^
0_0_27686067_765.cpp: In function 'int main()':
0_0_27686067_765.cpp:13:13: error: 'pos' was not declared in this scope
             pos[a[i]] = i;
             ^
0_0_27686067_765.cpp:15:20: error: 'sort' was not declared in this scope
         sort(a, a+n);
                    ^
0_0_27686067_765.cpp:15:20: note: suggested alternative:
In file included from GCC4.9.2/x86_64-w64-mingw32/include/c++/algorithm:62:0,
                 from 0_0_27686067_765.cpp:2:
GCC4.9.2/x86_64-w64-mingw32/include/c++/bits/stl_algo.h:4705:5: note:   'std::sort'
     sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
     ^
0_0_27686067_765.cpp:19:16: error: 'pos' was not declared in this scope
             if(pos[l] > pos[a[i]]) {
                ^