英文句子中各个单词的出现次数统计

#include <bits/stdc++.h>
using namespace std;
int main(){
    string a,b;
    map<string,int> a1;
    vector<string> a2;
    string max1;
    int max2;
    while(cin>>a){
        b = "";
        for(int i = 0;i<a.size();i++){
            if(a[i]>='a' && a[i]<='z' || a[i]>='A' && a[i]<='Z'){
                if(a[i]>='A' && a[i]<='Z'){
                    a[i] += 32;
                }
                b += a[i];
            }
        }
        if(a1[b]==0){
            a2.push_back(b);
        }
        a1[b]++;
        max2 = max(a1[b],max2);
        if(max2==a1[b]){
            max1 = b;
        }
    }
    for(int i = 0;i<a2.size();i++){
        cout<<a2[i]<<" "<<a1[a2[i]]<<endl;
    }
    cout<<max1<<" "<<max2;
    return 0;
}