Letter Combinations of a Phone Number LeetCode solution in C++

1
class Solution {
2
 public:
3
  void solve(string digit, string output, int index, vector<string> &ans,
4
             string mapping[]) { // base condition
5
    if (index >= digit.length()) {
6
      ans.push_back(output);
7
      return;
8
    }
9
    // digit[index] gives character value to change in integer subtract '0'
10
    int number = digit[index] - '0';
11
    // get the string at perticular index in mapping
12
    string value = mapping[number];
13
    // runs loop in value string and push that value in out put string ans do
14
    // recursive call for next index
15
    for (int i = 0; i < value.length(); i++) {
16
      output.push_back(value[i]);
17
      solve(digit, output, index + 1, ans, mapping);
18
      // backtrack
19
      // backtrach because initially output is empty and one case solves now you
20
      // have to solve second case in similar way
21
      output.pop_back();
22
    }
23
  }
24

25
  vector<string> letterCombinations(string digits) {
26
    vector<string> ans;
27
    // if it is empty input string
28
    if (digits.length() == 0) {
29
      return ans;
30
    }
31
    string output = "";
32
    int index = 0;
33
    // map every index with string
34
    string mapping[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
35
    solve(digits, output, index, ans, mapping);
36
    return ans;
37
  }
38
};