Merge pull request cpsumsu#1 from awepo-pro/main

update leetcode 17 and gitignore file

.gitignore

@@ -0,0 +1,35 @@
+# Prerequisites
+*.d
+
+# Compiled Object files
+*.slo
+*.lo
+*.o
+*.obj
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Compiled Dynamic libraries
+*.so
+*.dylib
+*.dll
+
+# Fortran module files
+*.mod
+*.smod
+
+# Compiled Static libraries
+*.lai
+*.la
+*.a
+*.lib
+
+# Executables
+*.exe
+*.out
+*.app
+
+# OS
+.DS_Store

leetcode/17.cpp

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+// https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/
+
+class Solution {
+public:
+    // use hash map store as well
+    const vector<string> pad = {
+        "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"
+    };
+
+    vector<string> letterCombinations(string digits) {
+        if (digits.empty()) return {};
+
+        vector<string> result;
+        // very important to append empty string, otherwise line 19 would not implement for empty vector
+        result.push_back("");
+
+        for (auto digit : digits) {
+            vector<string> tmp;
+
+            for (auto candidate : pad[digit - '2']) {
+                // for each element in result, append the new character 
+                for (auto s : result) {
+                    // store them into tmp so that tmp currently becomes the answer
+                    tmp.push_back(s + candidate);
+                }
+            }
+
+            result.swap(tmp);
+        }
+
+        return result;
+    }
+};