feat: 17_letter_combinations_of_a_phone_number

17_letter_combinations_of_a_phone_number/main.cpp

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+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+   public:
+    unordered_map<char, unordered_set<string>> charmap = {
+        {'2', {"a", "b", "c"}},
+        {'3', {"d", "e", "f"}},
+        {'4', {"g", "h", "i"}},
+        {'5', {"j", "k", "l"}},
+        {'6', {"m", "n", "o"}},
+        {'7', {"p", "q", "r", "s"}},
+        {'8', {"t", "u", "v"}},
+        {'9', {"w", "x", "y", "z"}},
+    };
+
+    unordered_set<string> combineTwo(const unordered_set<string> &a, const unordered_set<string> &b) {
+        unordered_set<string> result;
+
+        for (auto first : a) {
+            for (auto second : b) {
+                result.insert(first + second);
+            }
+        }
+
+        return result;
+    }
+
+    vector<string> letterCombinations(string digits) {
+        if (digits.length() == 0) return {};
+
+        unordered_set<string> resultSet = charmap[digits[0]];
+
+        for (int i = 1; i < digits.length(); i++) {
+            resultSet = combineTwo(resultSet, charmap[digits[i]]);
+        }
+
+        vector<string> result;
+        for (auto s : resultSet) result.emplace_back(s);
+
+        return result;
+    }
+};