Add C++ code for Problem 17

C++/17.cpp

@@ -0,0 +1,37 @@
+/* 
+    Problem 17. Letter Combinations of a Phone Number
+
+    Given a string containing digits from 2-9 inclusive,
+    return all possible letter combinations that the number could represent.
+    Return the answer in any order.
+
+    https://leetcode.com/problems/letter-combinations-of-a-phone-number/
+*/
+
+class Solution {
+public:
+    string mp[10] = {
+        "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"
+    };
+
+    void helper(int st, string digits, vector<string>& ans) {
+        if(st == digits.length()) {
+            ans.push_back(digits);
+            return;
+        }
+        char temp = digits[st];
+        string vals = mp[temp - '0'];
+        for(int i = 0; i < vals.length(); i++) {
+            digits[st] = vals[i];
+            helper(st+1, digits, ans);
+        }
+        digits[st] = temp;
+    }
+
+    vector<string> letterCombinations(string digits) {
+        vector<string> ans;
+        if(digits.length() > 0)
+            helper(0, digits, ans);
+        return ans;
+    }
+};