/**
5.4-函数的默认参数
*/
let str1 = joinString(s1: "hell",toString: "world")
print(str1)
let str2 = joinString(s1: "hell",toString: "world",joiner: "¥")
print(str2)
let str3 = joinString2(s1: "hellw",joiner: "#")
print(str3)
let str4 = joinString2(s1: "llll",toString: "jijiji")
print(str4)