i++和++i傻傻分不清

i++,++i都会使i加1
假设i = 0

a = i++ # a = 0, i = 1
a = ++i # a = 1, i = 1

简单概括：i++返回递增前的值，++i返回递增后的值

i++
a = i # a = 1, i = 1