foo.c:11:2: warning: missing braces around initializer [-Wmissing-braces]
  }D[maxn][maxn]={0};
  ^
foo.c:11:2: warning: (near initialization for 'D[0]') [-Wmissing-braces]
foo.c: In function 'main':
foo.c:27:24: error: incompatible types when assigning to type 'str' from type 'char'
  for (i=L-1;i>-1;i--) s=s[i];
                        ^