Python 2, (削除) 99 (削除ここまで) 98 bytes

exec"f,F="+"lambda n,i=0:n<2and i or %s"*2%("f([n/2,3*n+1][n%2],-~i),","i>99and'f'or F(f(n),-~i)")

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Python 2, (削除) 99 (削除ここまで) (削除) 98 (削除ここまで) 97 bytes

• Saved a byte by using c and t or f instead of t if c else f.
• Saved a byte by outputting -1 instead of f or 'f' for non-halting inputs.

exec"f,F="+"lambda n,i=0:n<2and i or %s"*2%("f([n/2,3*n+1][n%2],-~i),","i>99and-1or F(f(n),-~i)")

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Python 2, 99 bytes

exec"f,F="+"lambda n,i=0:i if n<2else f%s"*2%("([n/2,3*n+1][n%2],-~i),"," if i>99else F(f(n),-~i)")

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Python 2, (削除) 99 (削除ここまで) 98 bytes

exec"f,F="+"lambda n,i=0:n<2and i or %s"*2%("f([n/2,3*n+1][n%2],-~i),","i>99and'f'or F(f(n),-~i)")

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Python 2, 99 bytes

exec"f%s([n/2,3*n+1][n%%2],-~i)\nF%s if i>99else F(f(n),-~i)"%(2*("=lambda n,i=0:i if n<2else f",))

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Python 2, 99 bytes

exec"f,F="+"lambda n,i=0:i if n<2else f%s"*2%("([n/2,3*n+1][n%2],-~i),"," if i>99else F(f(n),-~i)")

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