As noted in the 2004 AB MC explanations, this problem is more appropriately solved using the technique presented in analyzing recursive methods. 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 and some of the material from the more complex stack based trace with nested recursive calls. Familiarity with the material from those resources is assumed, including the notation.

Step 1: Initial call

Initial call stack

m(2, 3)

Step 2: mystery(2, 3)

• Checks if 2 != 0, which is true, so the entire condition is true
• Computes mat[2][3], which is 11, + ____ (something unknown)
• Calls mystery(2 - 1, 3 - 1), which is mystery(1, 2)

Resulting call stack

m(1, 2)
m(2, 3)    11 + ____

Step 3: mystery(1, 2)

• Checks if 1 != 0, which is true, so the entire condition is true
• Computes mat[1][2], which is 6, + ____
• Calls mystery(1 - 1, 2 - 1), which is mystery(0, 1)

Resulting call stack

m(0, 1)
m(1, 2)    6 + ____
m(2, 3)    11 + ____

Step 4: mystery(0, 1)

• Checks if 0 != 0, which is false, or 1 != 0, which is true, so the entire condition is true
• Computes mat[0][1], which is 1, + ____
• Calls mystery(0 - 1, 1 - 1), which is mystery(-1, 0)

Resulting call stack

m(-1, 0)
m(0, 1)    1 + ____
m(1, 2)    6 + ____
m(2, 3)    11 + ____

Step 5: mystery(-1, 0)

• Checks if -1 != 0, which is true, so the entire condition is true
• Accesses mat[-1][0], which throws an ArrayIndexOutOfBoundsException