内推阿里电话面试中面试官给我出的一个题：
我想的头一个解决方案，就是放到stl 的map里面对出现的频率作为pair的第二个字段进行排序，之后按照排序结果返回：
下面口说无凭，show your code，当然在讨论帖子中遭遇了工程界大牛的sql代码在技术上的碾压。什么是做工程的，什么是工程师的思维，不要一味的埋头搞算法。
讨论帖：
http://bbs.csdn.net/topics/391080906
python 抓取百度搜索结果的讨论贴：
http://bbs.csdn.net/topics/391077668
实验数据，python从百度抓得：
# -*- coding: utf-8 -*-
"""
Spyder Editor
This is a temporary script file.
"""
import urllib2
import re
import os
#connect to a URL
#一页的搜索结果中url大概是200个左右
file_url = open('url.txt','ab+')
#搜索框里的东西,这块可以设置成数字好让每次搜索的结果不一样
search = '123'
url = "http://www.baidu.com/s?wd="+search
def setUrlToFile():
website = urllib2.urlopen(url)
#read html code
html = website.read()
#use re.findall to get all the links
links = re.findall('"((http|ftp)s?://.*?)"', html)
for s in links:
print s[0]
if len(s[0]) < 256:
file_url.write(s[0]+'
')
#收集实验数据
for i in range(0,50):
setUrlToFile()
file_url.close()
###需要重新打开再读一下
file_url = open('url.txt','r')
file_lines = len(file_url.readlines())
print "there are %d url in %s" %(file_lines,file_url)
file_url.close()
方法1：
c++ 写的读 url.txt放到map里面
对map<string , int>的value进行排序，得到前100个
运行一下也就55s，还是很快的，url长度进行了限制小于256个字符
#pragma once
/*
//计算代码段运行时间的类
//
*/
#include <iostream>
#ifndef ComputeTime_h
#define ComputeTime_h
//单位毫秒
class ComputeTime
{
private:
int Initialized;
__int64 Frequency;
__int64 BeginTime;
public:
bool Avaliable();
double End();
bool Begin();
ComputeTime();
virtual ~ComputeTime();
};
#endif
#include "stdafx.h"
#include "ComputeTime.h"
#include <iostream>
#include <Windows.h>
ComputeTime::ComputeTime()
{
Initialized=QueryPerformanceFrequency((LARGE_INTEGER *)&Frequency);
}
ComputeTime::~ComputeTime()
{
}
bool ComputeTime::Begin()
{
if(!Initialized)
return 0;
return QueryPerformanceCounter((LARGE_INTEGER *)&BeginTime);
}
double ComputeTime::End()
{
if(!Initialized)
return 0;
__int64 endtime;
QueryPerformanceCounter((LARGE_INTEGER *)&endtime);
__int64 elapsed = endtime-BeginTime;
return ((double)elapsed/(double)Frequency)*1000.0; //单位毫秒
}
bool ComputeTime::Avaliable()
{
return Initialized;
}
// sortUrl.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
//#include <utility>
#include <vector>
#include <map>
#include <fstream>
#include <iostream>
#include <string>
#include <algorithm>
#include "ComputeTime.h"
using namespace std;
map<string,int> urlfrequency;
typedef pair<string, int> PAIR;
struct CmpByValue
{
bool operator()(const PAIR& lhs, const PAIR& rhs)
{
return lhs.second > rhs.second;
}
};
void find_largeTH(map<string,int> urlfrequency)
{
//把map中元素转存到vector中 ,按照value排序
vector<PAIR> url_quency_vec(urlfrequency.begin(), urlfrequency.end());
sort(url_quency_vec.begin(), url_quency_vec.end(), CmpByValue());
//url_quency_vec.size()
for (int i = 0; i != 100; ++i)
{
cout<<url_quency_vec[i].first<<endl;
cout<<url_quency_vec[i].second<<endl;
}
}
//urlheap的建立过程，URL插入时候存在的
void insertUrl(string url)
{
pair<map<string ,int>::iterator, bool> Insert_Pair;
Insert_Pair = urlfrequency.insert(map<string, int>::value_type(url,1));
if (Insert_Pair.second == false)
{
(Insert_Pair.first->second++);
}
}
int _tmain(int argc, _TCHAR* argv[])
{
fstream URLfile;
char buffer[1024];
URLfile.open("url.txt",ios::in|ios::out|ios::binary);
if (! URLfile.is_open())
{ cout << "Error opening file"; exit (1); }
else
{
cout<<"open file success!"<<endl;
}
ComputeTime cp;
cp.Begin();
int i = 0;
while (!URLfile.eof())
{
URLfile.getline (buffer,1024);
//cout << buffer << endl;
string temp(buffer);
//cout<<i++<<endl;
insertUrl(temp);
}
find_largeTH(urlfrequency);
cout<<"running time: "<<cp.End()<<"ms"<<endl;
getchar();
//system("pause");
return 0;
}
实验结果：55s还不算太差，可以接受，毕竟是头脑中的第一个解决方案。
方法2：
hash code 版本，只是不知道怎么 hash和url关联起来：
// urlFind.cpp : 定义控制台应用程序的入口点。
//
// sortUrl.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <vector>
#include <map>
#include <fstream>
#include <iostream>
#include <string>
#include <algorithm>
#include <unordered_map>
#include "ComputeTime.h"
using namespace std;
map<unsigned int,int> urlhash;
typedef pair<unsigned int, int> PAIR;
struct info{
string url;
int cnt;
bool operator<(const info &r) const {
return cnt>r.cnt;
}
};
unordered_map<string,int> count;
//priority_queue<info> pq;
struct CmpByValue
{
bool operator()(const PAIR& lhs, const PAIR& rhs)
{
return lhs.second > rhs.second;
}
};
void find_largeTH(map<unsigned int,int> urlhash)
{
//把map中元素转存到vector中 ,按照value排序
vector<PAIR> url_quency_vec(urlhash.begin(), urlhash.end());
sort(url_quency_vec.begin(), url_quency_vec.end(), CmpByValue());
//url_quency_vec.size()
for (int i = 0; i != 100; ++i)
{
cout<<url_quency_vec[i].first<<endl;
cout<<url_quency_vec[i].second<<endl;
}
}
// BKDR Hash Function
unsigned int BKDRHash(char *str)
{
unsigned int seed = 131; // 31 131 1313 13131 131313 etc..
unsigned int hash = 0;
while (*str)
{
hash = hash * seed + (*str++);
}
return (hash & 0x7FFFFFFF);
}
//
void insertUrl(string url)
{
unsigned int hashvalue = BKDRHash((char *)url.c_str());
pair<map<unsigned int ,int>::iterator, bool> Insert_Pair;
Insert_Pair = urlhash.insert(map<unsigned int, int>::value_type(hashvalue,1));
if (Insert_Pair.second == false)
{
(Insert_Pair.first->second++);
}
}
int _tmain(int argc, _TCHAR* argv[])
{
fstream URLfile;
char buffer[1024];
URLfile.open("url.txt",ios::in|ios::out|ios::binary);
if (! URLfile.is_open())
{ cout << "Error opening file"; exit (1); }
else
{
cout<<"open file success!"<<endl;
}
ComputeTime cp;
cp.Begin();
int i = 0;
while (!URLfile.eof())
{
URLfile.getline (buffer,1024);
//cout << buffer << endl;
string temp(buffer);
//cout<<i++<<endl;
insertUrl(temp);
}
find_largeTH(urlhash);
cout<<"running time: "<<cp.End()<<"ms"<<endl;
getchar();
//system("pause");
return 0;
}
性能15秒左右：缺点在于没有把hashcode和url进行关联，技术的处理速度已经非常可观了
方法3：
下面用STL的hash容器unordered_map，和优先队列(就是堆)来实现这个问题。
// urlFind.cpp : 定义控制台应用程序的入口点。
//
// sortUrl.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <vector>
#include <map>
#include <fstream>
#include <iostream>
#include <string>
#include <algorithm>
#include <unordered_map>
#include <queue>
#include "ComputeTime.h"
using namespace std;
typedef pair<string, int> PAIR;
struct info
{
string url;
int cnt;
bool operator<(const info &r) const
{
return cnt<r.cnt;
}
};
unordered_map<string,int> hash_url;
priority_queue<info> pq;
void find_largeTH(unordered_map<string,int> urlhash)
{
unordered_map<string,int>::iterator iter = urlhash.begin();
info temp;
for (; iter!= urlhash.end();++iter)
{
temp.url = iter->first;
temp.cnt = iter->second;
pq.push(temp);
}
for (int i = 0; i != 100; ++i)
{
cout<<pq.top().url<<endl;
cout<<pq.top().cnt<<endl;
pq.pop();
}
}
void insertUrl(string url)
{
pair<unordered_map<string ,int>::iterator, bool> Insert_Pair;
Insert_Pair = hash_url.insert(unordered_map<string, int>::value_type(url,1));
if (Insert_Pair.second == false)
{
(Insert_Pair.first->second++);
}
}
int _tmain(int argc, _TCHAR* argv[])
{
fstream URLfile;
char buffer[1024];
URLfile.open("url.txt",ios::in|ios::out|ios::binary);
if (! URLfile.is_open())
{ cout << "Error opening file"; exit (1); }
else
{
cout<<"open file success!"<<endl;
}
ComputeTime cp;
cp.Begin();
int i = 0;
while (!URLfile.eof())
{
URLfile.getline (buffer,1024);
//cout << buffer << endl;
string temp(buffer);
//cout<<i++<<endl;
insertUrl(temp);
}
find_largeTH(hash_url);
cout<<"running time: "<<cp.End()<<"ms"<<endl;
getchar();
//system("pause");
return 0;
}
基本上算是算法里面比较优秀的解决方案了，面试官如果能听到这个方案应该会比较欣喜。
方法4：实验耗时未知，技术上碾压了上述解决方案，中高年轻人，不要重复造轮子！哈哈
数据库，SQL语句：
load data infile "d:/bigdata.txt" into table tb_url(url); SELECT url, count(url) as show_count FROM tb_url GROUP BY url ORDER BY show_count desc LIMIT 100