示例:1+2+3+4+5 = 15
def fun(n): if n == 0: return 0 else: return n + fun(n+1) fun(1)
def fun(n): if n == 0: return 0 else: return n + fun(n-1) fun(5)
def fun(n): if n == 5: return 5 else: return n + fun(n+1) fun(5)
def fun(n): if n == 5: return 5 else: return n + fun(n-1) fun(1)