Add return 0 at the end of excution.

1/homework/exp1/exp1_3.asm

@@ -62,5 +62,6 @@ output:
     j       output
 
 done_output:
+    li      $a0,            0
     li      $v0,            17
     syscall 

1/homework/exp1/exp1_4.asm

@@ -10,6 +10,7 @@ main:
     move    $a0,            $v0                     # print out
     li      $v0,            1
     syscall 
+    li      $a0,            0
     li      $v0,            17
     syscall 
 

1/homework/exp2/binary_insert_sort.asm

@@ -61,6 +61,7 @@ main:
     li      $v0,                15
     syscall 
 
+    li      $a0,                0
     li      $v0,                17
     syscall                                                                     # exit
 

1/homework/exp2/insert_sort.asm

@@ -58,6 +58,7 @@ main:
     li      $v0,                15
     syscall 
 
+    li      $a0,                0
     li      $v0,                17
     syscall                                                             # exit
 

1/homework/exp2/merge_sort.asm

@@ -97,6 +97,7 @@ done_write_linked_list:
     li      $v0,                    16
     syscall 
 
+    li      $a0,                    0
     li      $v0,                    17
     syscall                                                                         # exit