【今日三题】跳台阶扩展问题(找规律) / 包含不超过两种字符的最长子串 / 字符串的排列(回溯—全排列)

#include <iostream>
using namespace std;

int main()
{
    int n;
    cin >> n;
    cout << (1 << (n - 1)) << endl;
    return 0;
}

#include <iostream>
#include <string>
using namespace std;

string s;
int len;
int Hash[26];

int main() 
{
    cin >> s;
    for (int l = 0, r = 0, cnt = 0; r < s.size(); r++)
    {
        if (Hash[s[r] - 'a']++ == 0) cnt++;
        while (cnt > 2)
        {
            if (--Hash[s[l++] - 'a'] == 0) cnt--;
        }
        len = max(len, r - l + 1);
    }
    cout << len << endl;
    return 0;
}

class Solution {
    vector<string> res;
    string path;
    bool used[11] = {};
public:
    vector<string> Permutation(string str) {
        sort(str.begin(), str.end());
        dfs(str);
        return res;
    }
    void dfs(const string &str)
    {
        if (path.size() == str.size())
        {
            res.push_back(path);
            return;
        }
        for (int i = 0; i < str.size(); i++)
        {
            if (!used[i] && (i == 0 || str[i - 1] != str[i] || used[i - 1]))
            {
                used[i] = true;
                path += str[i];
                dfs(str);
                path.pop_back();
                used[i] = false;
            }
        }
    }
};