直接计数

$O(|s|)$

#include <bits/stdc++.h>
using namespace std;
string s;
int cnt[26][26];
int main()
{
    cin >> s;
    int maxCnt = 0;
    for (int i = 0; i < s.length() - 1; i++)
    {
        cnt[s[i] - 'a'][s[i + 1] - 'a']++;
        maxCnt = max(maxCnt, cnt[s[i] - 'a'][s[i + 1] - 'a']);
    }
    for (int i = 0; i < 26; i++)
        for (int j = 0; j < 26; j++)
            if (cnt[i][j] == maxCnt)
                cout << (char)(i + 'a') << (char)(j + 'a') << "\n";
    return 0;
}

STL map

#include <bits/stdc++.h>
using namespace std;
string s;
// m[str] 统计 str 的出现次数
map<string, int> m;
int main()
{
    cin >> s;
    int maxCnt = 0;
    for (int i = 0; i < s.length() - 1; i++)
    {
        m[s.substr(i, 2)]++;
        maxCnt = max(maxCnt, m[s.substr(i, 2)]);
    }
    //可以这样写
    for (map<string, int>::iterator it = m.begin(); it != m.end(); it++)
        if ((*it).second == maxCnt)
            cout << (*it).first << "\n";

    //也可以这样写
    for (auto it = m.begin(); it != m.end(); it++)
        if (it->second == maxCnt)
            cout << it->first << "\n";

    //还可以这样写
    for (auto it : m)
        if (it.second == maxCnt)
            cout << it.first << "\n";
    
    return 0;
}