[itint5]单词变换

http://www.itint5.com/oj/#42

基本上就是word ladder。直接来BFS，记录前驱。

vector<string> transform(set<string> &dict, string from, string to) {

    vector<string> ans;

    if (from.length() != to.length()) {

        return ans;

    }

    queue<string> que;

    map<string, string> prev_map;

    que.push(from);

    prev_map[from] = "";

    while (!que.empty()) {

        string s = que.front();

        que.pop();

        if (s == to) break;

        for (int i = 0; i < s.length(); i++) {

            char tmp = s[i];

            for (char c = 'A'; c <= 'Z'; c++) {

                if (tmp == c) continue;

                s[i] = c;

                if (dict.find(s) != dict.end()

                    && prev_map.find(s) == prev_map.end()) {

                    que.push(s);

                    prev_map[s] = s;

                    prev_map[s][i] = tmp;

                }

            }

            s[i] = tmp;

        }

    }

    if (prev_map.find(to) == prev_map.end()) return ans;

    string last = to;

    do {

        ans.push_back(last);

        last = prev_map[last];

    } while (last != "");

    for (int i = 0; i < ans.size() / 2; i++) {

        int k = ans.size() - i - 1;

        string tmp = ans[i];

        ans[i] = ans[k];

        ans[k] = tmp;

    }

    return ans;

}