python solution:

使用defaultdict来做，非常easy

from collections import defaultdict
while True:
    try:
        dd = defaultdict(list)
        for _ in range(int(input())):
            bookName, typeName = input().split()
            dd[typeName].append(bookName)
        for i in sorted(dd.keys(),key=lambda c:c.upper()):
            for j in sorted(dd[i],key=lambda c:c.upper()):
                print (j)
        print ()
    except:
        break

#include<iostream>
#include<vector>
#include<string>
#include<algorithm>
#include<functional>
#include <map>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <exception>
#include <iomanip>
#include <memory>
#include <sstream>

#define INF 1000000
using namespace std;

typedef pair<string, string> StrPair;
int main(int argc, char** argv)
{
	//freopen("in.txt", "r", stdin);
	int n;
	while (cin >> n)
	{
		string name, type;
		vector<StrPair> shelf;
		unordered_map<string, string> hs;
		shelf.reserve(n);
		for (int i = 0; i < n; ++i)
		{
			cin >> name >> type;
			string lower_name = name;
			for (auto& c : lower_name) c = tolower(c);
			for (auto& c : type) c = tolower(c);
			hs.emplace(lower_name, name);
			shelf.emplace_back(lower_name, type);
		}
		sort(shelf.begin(), shelf.end(),
			[](const StrPair& o1, const StrPair& o2)
			{
				return o1.second < o2.second || o1.second == o2.second && o1.first < o2.first;
			});

		for (auto& pr : shelf) cout << hs[pr.first] << endl;
		cout << endl;
	}
	return 0;
}

import java.util.*;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while (sc.hasNext()) {
			int n = sc.nextInt();
			Book[] books = new Book[n];
			for (int i = 0; i < books.length; i ++ )
				books[i] = new Book(sc.next(), sc.next());
			Arrays.sort(books);
			for (Book book:books)
				System.out.println(book.name);
			System.out.println();
		}
	}
	static class Book implements Comparable<Book> {
		String name;
		String category;
		public Book(String name, String category) {
			this.name = name;
			this.category = category;
		}
		@Override
		public int compareTo(Book o) {
			int k = this.category.compareToIgnoreCase(o.category);
			if(k == 0) return this.name.compareToIgnoreCase(o.name);
			return k;
		}
	}
}

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
using namespace std;
typedef pair<string, string> PAIR;
bool cmp(const PAIR &x, const PAIR &y)
{
	return x.first < y.first;
}

typedef struct
{
	string classname;
	vector<PAIR> classdata;
	void nodesort()
	{
		sort(classdata.begin(), classdata.end(), cmp);
	}
}node;

int main()
{
	int length;
	string bookname, bookclass;
	while (cin >> length)
	{
		map<string, node> data;
		
		for (int i = 0; i < length; i++)
		{
			cin >> bookname >> bookclass;
			string firstname;
			for (auto c : bookname)
				firstname += tolower(c);
			
			for (auto &c : bookclass)
				c = tolower(c);
			PAIR p = make_pair(firstname, bookname);
			data[bookclass].classdata.push_back(p);
		}
		for (map<string, node>::iterator it = data.begin(); it != data.end(); it++)
		{
			it->second.nodesort();
			for (vector<PAIR>::iterator k = it->second.classdata.begin(); k != it->second.classdata.end(); k++)
				cout << k->second << endl;		
		}
		cout << endl;
	}
	return 0;
}

#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <set>
using namespace std;

class bookSort : public less<string>		// 定义升序比较器
{
public:										// 定义大小写无关的比较器
	bool operator()(string a, string b) const 
	{
		for (char & ch : a) ch = tolower(ch);
		for (char & ch : b) ch = tolower(ch);
		return a < b;						// 返回字典序
	}
};

vector<string> solve(map<string, set<string, bookSort>, bookSort>  & store)
{
	vector<string> list;

	// 对于mao中的每一个key(目录名)值，输出对应的所有value(书名)。
	for (auto pairs : store)
	{
		string category = pairs.first;		// 目录名
		for (auto name : store[category])	// 每个书名
			list.push_back(name);			// 将书名存入到最终输出列表内
	}
	return list;							// 输出列表返回给main函数
}

int main()
{
	int n, flag = 0;
	while (cin >> n && n)
	{
		map<string, set<string, bookSort>, bookSort> store;
		string name, category;
		for (int i = 0; i < n; ++i)
		{
			cin >> name >> category;		// 读入目录名和书名
			store[category].insert(name);	// 根据目录名，将书名插入到 set 集合
		}

		vector<string> list = solve(store);	// 获取处理后的书名列表
		for (auto str : list)				// 输出所有书名
			cout << str << endl;
		cout << endl;
	}
	return 0;
}