阶乘求和 0!+1!+2!+3!+4!+5!+... O(n) 复杂度

n次循环 以 n = 4 为例ios 利用 n!+(n-1)! = (n + 1) x ( n - 1)! 4! + 3! + 2! + 1! + 0! = (4+1) x 3! + 2! + 1! + 0! = ( (4+1) x 3 + 1) x 2! + 1! + 0! = ( ( (4+1) x 3 + 1) x 2 + 1) x 1 + 0! = ( ( ( (4+1) x 3 +

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