foo.c:11:6: error: conflicting types for 'heshu'
 void heshu(int l,int r)
      ^~~~~
foo.c:5:8: note: previous declaration of 'heshu' was here
    int heshu(int l,int r);
        ^~~~~
foo.c: In function 'main':
foo.c:7:4: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d %d",&l,&r);//先小后大
    ^~~~~~~~~~~~~~~~~~~~

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