了解堆栈的工作原理
var homeworkStack = ["BIO12","HIS80","MAT122","PSY44"];
// Only change code below this line
homeworkStack.pop();
homeworkStack.push('CS50')
var homeworkStack = ["BIO12","HIS80","MAT122","PSY44"];
// Only change code below this line
homeworkStack.pop();
homeworkStack.push('CS50')