foo.c: In function 'SetUnion':
foo.c:15:5: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
   15 |     for(;j<k;j++)
      |     ^~~
foo.c:17:6: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'for'
   17 |      k--;
      |      ^
foo.c:23:1: warning: control reaches end of non-void function [-Wreturn-type]
   23 | }
      | ^
foo.c: In function 'main':
foo.c:30:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   30 |  scanf("%d%d%d",&n1,&n2,&n3);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:33:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   33 |   scanf("%d",&a1[i]);
      |   ^~~~~~~~~~~~~~~~~~
foo.c:35:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   35 |   scanf("%d",&a2[i]);
      |   ^~~~~~~~~~~~~~~~~~
foo.c:37:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   37 |   scanf("%d",&a3[i]);
      |   ^~~~~~~~~~~~~~~~~~

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