As noted in the 2009 A MC explanations, this problem is more appropriately solved using the technique presented in recursive methods with print statements. The trace below is presented as a demonstration of tracing. It is not a suggestion that this problem should be traced.

This trace uses the technique demonstrated in tracing recursive methods. Familiarity with that material is assumed.

Step 1: initial call

Initial call stack

m(123456)

Step 2: mystery(123456)

• Checks if 123456 / 10, which is 12345, is != 0, which is true
• Computes 123456 / 10, which is 12345
• Calls mystery(12345)

Resulting call stack

m(12345)
m(123456)

Step 3: mystery(12345)

• Checks if 12345 / 10, which is 1234, is != 0, which is true
• Computes 12345 / 10, which is 1234
• Calls mystery(1234)

Resulting call stack

m(1234)
m(12345)
m(123456)

Step 4: mystery(1234)

• Checks if 1234 / 10, which is 123, is != 0, which is true
• Computes 1234 / 10, which is 123
• Calls mystery(123)

Resulting call stack

m(123)
m(1234)
m(12345)
m(123456)

Step 5: mystery(123)

• Checks if 123 / 10, which is 12, is != 0, which is true
• Computes 123 / 10, which is 12
• Calls mystery(12)

Resulting call stack

m(12)
m(123)
m(1234)
m(12345)
m(123456)

Step 6: mystery(12)

• Checks if 12 / 10, which is 1, is != 0, which is true
• Computes 12 / 10, which is 1
• Calls mystery(1)

Resulting call stack

m(1)
m(12)
m(123)
m(1234)
m(12345)
m(123456)

Step 7: mystery(1)

• Checks if 1 / 10, which is 0, is != 0, which is false
• Computes 1 % 10, which is 1
• Prints 1
• Returns (ends)

Resulting call stack

m(1)  returns
m(12)
m(123)
m(1234)
m(12345)
m(123456)

Printed

1

Step 8: Back in mystery(12)

• Back in mystery(12)
• Finished the recursive call
• Computes 12 % 10, which is 2
• Prints 2
• Returns

Resulting call stack

m(1)  returns
m(12)  returns
m(123)
m(1234)
m(12345)
m(123456)

Printed

12

Step 9: Back in mystery(123)

• Back in mystery(123)
• Finished the recursive call
• Computes 123 % 10, which is 3
• Prints 3
• Returns

Resulting call stack

m(1)  returns
m(12)  returns
m(123)  returns
m(1234)
m(12345)
m(123456)

Printed

123

Step 10: Back in mystery(1234)

• Back in mystery(1234)
• Finished the recursive call
• Computes 1234 % 10, which is 4
• Prints 4
• Returns

Resulting call stack

m(1)  returns
m(12)  returns
m(123)  returns
m(1234)  returns
m(12345)
m(123456)

Printed

1234

Step 11: Back in mystery(12345)

• Back in mystery(12345)
• Finished the recursive call
• Computes 12345 % 10, which is 5
• Prints 5
• Returns

Resulting call stack

m(1)  returns
m(12)  returns
m(123)  returns
m(1234)  returns
m(12345)  returns
m(123456)

Printed

12345

Step 12: Back in mystery(123456)

• Back in mystery(123456)
• Finished the recursive call
• Computes 123456 % 10, which is 6
• Prints 6
• Returns

Resulting call stack

m(1)  returns
m(12)  returns
m(123)  returns
m(1234)  returns
m(12345)  returns
m(123456)  returns

Printed

123456