小米字符串第一题，状态机思路

这题用状态机比较好解决，代码看起来清晰很多

#include <iostream>
#include <vector>
#include <string>
#include <list>
#include <queue>
#include <unordered_map>
#include <algorithm>

using namespace std;

string to_upper(string& s) {
    for (auto& c : s) {
        c = toupper(c);
    }
    return s;
}

int main() {
    string s;
    while (cin >> s) {
        int state = 0;
        vector<string> v;

        int i, j;

        for (i = 0, j = 0; j < s.length(); j++) {
            if (s[j] == '.') {
                state = 0;
                v.push_back(s.substr(i, j - i));
                i = j + 1;
            }
            else if (isupper(s[j])) {
                if (state == 0) state = 1;
                else if (state == 1) state = 3;
                else if (state == 2 || state == 4 || state == 5) {
                    v.push_back(s.substr(i, j - i));
                    state = 1;
                    i = j;
                }
            }
            else if (islower(s[j])) {
                if (state == 0) state = 5;
                else if (state == 1) state = 2;
                else if (state == 3) {
                    v.push_back(s.substr(i, j - i - 1));
                    state = 2;
                    i = j - 1;
                }
                else if (state == 4) {
                    v.push_back(s.substr(i, j - i));
                    state = 5;
                    i = j;
                }
            }
            else if (isdigit(s[j])) {
                if (state == 0) state = 4;
                else if (state != 4) {
                    v.push_back(s.substr(i, j - i));
                    state = 4;
                    i = j;
                }
            }
        }

        if (state != 0) v.push_back(s.substr(i, j - i));

        string res = "_";
        for (auto& e : v) {
            res += to_upper(e), res += "_";
        }
        cout << res << endl;
    }

    return 0;
}