foo.c:3:9: error: variably modified 'u' at file scope
    3 | int n,i,u[N],v[N],top,k;
      |         ^
foo.c:3:14: error: variably modified 'v' at file scope
    3 | int n,i,u[N],v[N],top,k;
      |              ^
foo.c:4:6: error: variably modified 'c' at file scope
    4 | char c[N],sta[N],ans[2*N];
      |      ^
foo.c:4:11: error: variably modified 'sta' at file scope
    4 | char c[N],sta[N],ans[2*N];
      |           ^~~
foo.c:4:18: error: variably modified 'ans' at file scope
    4 | char c[N],sta[N],ans[2*N];
      |                  ^~~

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