代码随想录算法训练营第二十五天| 216.组合总和III、17.电话号码的字母组合

216.组合总和III

题目链接
文章讲解

class Solution {
public:
    vector<vector<int>> ans;
    vector<int> group;

    void backtracking(int start, int count, const int& k) {
        if (group.size() == k) {
            if (count == 0) ans.push_back(group);
            return;
        }
        int end = 9 - (k - group.size()) + 1;
        for (int i = start; i <= end; i++) {
            count -= i;
            if (count < 0) break;
            group.push_back(i);
            backtracking(i + 1, count, k);
            group.pop_back();
            count += i;
        }
    }

    vector<vector<int>> combinationSum3(int k, int n) {
        backtracking(1, n, k);
        return ans;
    }
};

17.电话号码的字母组合

题目链接
文章讲解

class Solution {
public:
    const string letterMap[9] = {
        "",
        "abc",
        "def",
        "ghi",
        "jkl",
        "mno",
        "pqrs",
        "tuv",
        "wxyz",
    };

    void backtracking(int digitsIdx, string path, const string& digits, vector<string>& ans) {
        if (digitsIdx == digits.size()) {
            ans.push_back(path);
            return;
        }
        string letters = letterMap[digits[digitsIdx] - '1'];
        for (int i = 0; i < letters.size(); i++) 
            backtracking(digitsIdx + 1, path + letters[i], digits, ans);
    }

    vector<string> letterCombinations(string digits) {
        if (digits == "") return {};
        vector<string> ans;
        backtracking(0, "", digits, ans);
        return ans;
    }
};