17 - Letter Combinations of a Phone Number

 JAVA

• class Solution {
      public List<String> letterCombinations(String digits) {
          List<String> ans = new ArrayList<>();
          if (digits.length() == 0) {
              return ans;
          }
          ans.add("");
          String[] d = new String[] {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
          for (char i : digits.toCharArray()) {
              String s = d[i - '2'];
              List<String> t = new ArrayList<>();
              for (String a : ans) {
                  for (String b : s.split("")) {
                      t.add(a + b);
                  }
              }
              ans = t;
          }
          return ans;
      }
  }

C++

• class Solution {
  public:
      vector<string> letterCombinations(string digits) {
          if (digits.empty()) {
              return {};
          }
          vector<string> d = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
          vector<string> ans = {""};
          for (auto& i : digits) {
              string s = d[i - '2'];
              vector<string> t;
              for (auto& a : ans) {
                  for (auto& b : s) {
                      t.push_back(a + b);
                  }
              }
              ans = move(t);
          }
          return ans;
      }
  };