C ++中子字符串中字符频率的查询
在这个问题上，我们得到一个字符串。Q查询每个都有两个整数l和r以及字符ch。我们的任务是创建一个程序来解决C++中子字符串中字符频率的查询。
问题描述：在这里，对于每个查询，我们将在子字符串str[l...r]中找到字符“ch”的出现频率。
让我们举个例子来了解这个问题，
输入值
str = “nhooo” Q = 2 0 6 t 5 13 i
输出结果
2 2
说明
对于查询1-子字符串为“tutoria”，字符t出现2次。
对于查询2-子字符串为“ialspoint”，字符i出现2次
解决方法
解决该问题的简便方法是将每个Querry的弦从l遍历到r，并计算弦中ch的出现。
该程序说明了我们解决方案的工作原理，
示例
#include <bits/stdc++.h>
using namespace std;
struct Query{
int l, r;
char ch;
};
int CalcCharFreq(string str, Query queries){
int count = 0;
for(int i = queries.l; i < queries.r; i++){
if(str[i] == queries.ch )
count++;
}
return count;
}
int main(){
string str = "nhooo";
int Q = 2;
Query queries[Q];
queries[0].l = 0;
queries[0].r = 5;
queries[0].ch = 't';
queries[1].l = 5;
queries[1].r = 13;
queries[1].ch = 'i';
for(int i = 0; i<Q; i++)
cout<<"For Query "<<(i+1)<<": The frequency of occurrence of character '"<<queries[i].ch<<"' is "<<CalcCharFreq(str, queries[i])<<"\n";
return 0;
}输出结果
For Query 1: The frequency of occurrence of character 't' is 2 For Query 2: The frequency of occurrence of character 'i' is 2
解决问题的另一种方法是使用预先计算的数组。在这里，我们将创建一个2D数组，该数组将存储直到特定索引的字符频率，即freq[3][2]将在索引2处存储字符'c'的频率。最初，所有频率均为0。
然后，我们将预先计算每个索引值处字符的频率。之后，我们通过从索引r处减去索引l处的出现频率，来找到该字符的频率。
让我们举个例子来了解这个问题，
示例
#include <bits/stdc++.h>
using namespace std;
int charFreq[100][26];
struct Query{
int l, r;
char ch;
};
void countCharFreq(string str, int size){
memset(charFreq, 0, sizeof(int));
for (int i = 0; i < size; i++){
charFreq[i][str[i] - 'a']++;
}
for (int i = 1; i < size; i++) {
for (int j = 0; j < 26; j++)
charFreq[i][j] += charFreq[i - 1][j] ;
}
}
int CalcCharFreq(Query queries){
return charFreq[queries.r][queries.ch - 'a'] - charFreq[queries.l][queries.ch - 'a'];
}
int main(){
string str = "nhooo";
int size = str.length();
int Q = 2;
countCharFreq(str, size);
Query queries[Q];
queries[0].l = 1;
queries[0].r = 13;
queries[0].ch = 't';
queries[1].l = 4;
queries[1].r = 13;
queries[1].ch = 'i';
for(int i = 0; i<Q; i++)
cout<<"For Query "<<(i+1)<<": The frequency of occurrence of character '"<<queries[i].ch<<"' is " <<CalcCharFreq( queries[i])<<"\n";
return 0;
}输出结果
For Query 1: The frequency of occurrence of character 't' is 2 For Query 2: The frequency of occurrence of character 'i' is 2