tsinghua university principle of compiler decaf programming assignment 1

使用lex和yacc写的Decaf语言的编译器前段代码，包括词法分析与语法分析。 Decaf是一门类Java的教学语言，这是THU2015年的PA-1(Written using lex and yacc compiler front-end code, including lexical analysis and syntax analysis. This is a PA from THU in 2015.)