Infix Notation
中缀表达式 是运算表达式的一种形式，运算符在操作数中间，比如 2 + 2。
常用于算术和逻辑公式和语句中，特点是人类可读、依赖运算符优先级。
Wikipedia
Infix notation is the notation commonly used in arithmetical and logical formulae and statements. It is characterized by the placement of operators between operands—"infixed operators"—such as the plus sign in 2 + 2.[1]