recursion.scala

def factorial(n: Int): Int = if (n == 0) 1 else n * factorial(n - 1)


factorial(4) =
if (4 == 0) 1 else 4 * factorial(4 - 1) = 
4 * factorial(4 - 1) =
4 * factorial(3) = 
4 * { if (3 == 0) 1 else 3 * factorial(3 - 1) } =
4 * { 3 * factorial(3 - 1) } =
4 * { 3 * factorial(2) } =
4 * { 3 * {if (2 == 0) 1 else 2 * factorial(2 - 1)} } = 
4 * { 3 * {2 * factorial(2 - 1)} } = 
4 * { 3 * {2 * factorial(1)} } = 
4 * { 3 * {2 * { if (1 == 0) 1 else 1 * factorial(1 - 1) } } } =
4 * { 3 * {2 * { 1 * factorial(1 - 1) } } } = 
4 * { 3 * {2 * { 1 * factorial(0) } } } = 
4 * { 3 * {2 * { 1 * { if (0 == 0) 1 else 0 * factorial(0 - 1) }  } } } = 
4 * { 3 * {2 * { 1 * { 1 }  } } } =
4 * { 3 * {2 * { 1 } } } = 
4 * { 3 * {2} } =
4 * { 6 } = 
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