leetcode题目例题解析（十三）

leetcode题目例题解析（十三）

Letter Combinations of a Phone Number

题目描述：

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].

题意解析：

这道题就是一个简单的递归解决排列组合的问题，注意的就是数字与字幕之间的对应转换

解题思路：

用递归的方式来解决，每一层处理一个数字所对应的情况

代码：

class Solution {
public:
  char f(int d, int m) {
    int k = d - '0';
    if (k >= 8)
      m += 1;//7对应的字母有四个，所以要+1
    return 'a' + (k-2)*3 + m;
  }
  void solve(string& digits, string pre, int deep, int length, vector<string>& ans)  {
    if (deep == length) {
      ans.push_back(pre);
    } else {
      string temp = pre;
      int end;
      if (digits[deep] == '9'||digits[deep] == '7')
    end = 4;
      else
    end = 3;
      for (int i = 0; i < end; i++) {
    char s = f(digits[deep], i);
    temp = pre;
    temp += s;
    solve(digits, temp, deep + 1, length, ans);
      }
    }
  }
  vector<string> letterCombinations(string digits) {
    int length = digits.length();
    vector<string> ans;
    string pre;
    if (length > 0)
    solve(digits, pre, 0, length, ans);
    return ans;
  }
};

原题目链接:
https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/