PAT甲级真题 1071 Speech Patterns (25分) C++实现（cctype判断字符属性、stringstream分割字符串、map计数）

题目

People often have a preference among synonyms of the same word. For example, some may prefer “the police”, while others may prefer “the cops”. Analyzing such patterns can help to narrow down a speaker’s identity, which is useful when validating, for example, whether it’s still the same person behind an online avatar.

Now given a paragraph of text sampled from someone’s speech, can you find the person’s most commonly used word?

Input Specification:

Each input file contains one test case. For each case, there is one line of text no more than 1048576 characters in length, terminated by a carriage return ‘\n’. The input contains at least one alphanumerical character, i.e., one character from the set [0-9 A-Z a-z].

Output Specification:

For each test case, print in one line the most commonly occurring word in the input text, followed by a space and the number of times it has occurred in the input. If there are more than one such words, print the lexicographically smallest one. The word should be printed in all lower case. Here a “word” is defined as a continuous sequence of alphanumerical characters separated by non-alphanumerical characters or the line beginning/end.

Note that words are case insensitive.

Sample Input:
Can1: “Can a can can a can? It can!”
Sample Output:
can 5

思路

用getline读入整行字符串，先从头到尾处理一遍：

1. 将所有字母变成小写；
2. 将所有非字母数字变成空格；

再用stringstream从上面处理过的字符串中读入一个个字符串，即得到了不区分大小写的单词，同时用unordered_map对单词计数.

最后遍历unordered_map找出最大数量和最大单次即可。

---

判断是否是字母或数字，以及转为小写字符，使用cctype.h中的库函数：

#include <cctype>

bool isalnum(char ch);  //如果参数是字母数字，即字母或者数字，函数返回true
char tolower(char ch);  //如果参数是大写字符，返回其小写，否则返回该参数

stringstream用于分割长字符串十分方便，用法为：

#include <sstream>
	
stringstream ss(s);  //以s为输入源建立流ss
string str;
while (ss >> str){  //读入ss中的各个字符串
        ...
}

代码

#include <iostream>
#include <cctype>
#include <sstream>
#include <unordered_map>
using namespace std;

int main(){
    string s;
    getline(cin, s);
    for (int i=0; i<s.size(); i++){
        if (isalnum(s[i])){
            s[i] = tolower(s[i]);
        }
        else{
            s[i] = ' ';
        }
    }
    unordered_map<string, int> mm;
    stringstream ss(s);
    while (ss >> s){
        mm[s]++;
    }
    int maxNum = 0;
    for (auto it=mm.begin(); it!=mm.end(); ++it){
        if (it->second > maxNum){
            s = it->first;
            maxNum = it->second;
        }
    }
    cout << s << " " << maxNum << endl;
    return 0;
}