LeetCode HOT100 - 电话号码的字母组合

就是回溯

每一轮选择可能的字母填上

class Solution {
public:
    vector<string> letterCombinations(string a) {
        vector<string> ans;
        string path;
        vector<string> mp = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        auto dfs = [&](this auto&& self, int u){
            if (u == a.size()) {
                ans.push_back(path);
                return;
            }
            for (char c : mp[a[u] - '0']) {
                path.push_back(c);
                self(u + 1);
                path.pop_back();
            }
        };
        if (a.empty()) {
            return {};
        }
        dfs(0);
        return ans;
    }
};