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0017-letter-combinations-of-a-phone-number/0017-letter-combinations-of-a-phone-number.cpp

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+class Solution {
+public:
+    vector<string> letterCombinations(string digits) {
+        vector<string> ans;
+        if(digits.size() == 0) 
+            return ans;
+        else if(digits.size() == 1)
+        {
+            if(digits[0] == '2')
+            {
+                return {"a", "b", "c"};
+            }
+            else if(digits[0] == '3'){
+                return {"d", "e", "f"};
+            }
+            else if(digits[0] == '4'){
+                return {"g", "h", "i"};
+            }
+            else if(digits[0] == '5'){
+                return {"j", "k", "l"};
+            }
+            else if(digits[0] == '6'){
+                return {"m", "n", "o"};
+            }
+            else if(digits[0] == '7'){
+                return {"p", "q", "r", "s"};
+            }
+            else if(digits[0] == '8'){
+                return {"t", "u", "v"};
+            }
+            else if(digits[0] == '9'){
+                return {"w", "x", "y", "z"};
+            }
+        }
+        else if(digits.size() > 1){
+            vector<string> p1 = letterCombinations(digits.substr(0, 1));
+            vector<string> p2 = letterCombinations(digits.substr(1, digits.size()-1));
+            for(int i = 0; i < p1.size(); i++)
+                for(int j = 0; j < p2.size(); j++)
+                    ans.push_back(p1[i] + p2[j]);
+        }
+        return ans;
+    }
+};