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

关于回溯的练习：

77. 组合

class Solution {
public:
    vector<vector<int>> paths;
    vector<int> path;
    void backtracking(int n, int k, int startIndex) {
        if(path.size()==k) {
            paths.push_back(path);
            return;
        }
        for(int i=startIndex; i<=n; ++i) {
            path.push_back(i);
            backtracking(n, k, i+1);
            path.pop_back();
        }
 
    }
 
    vector<vector<int>> combine(int n, int k) {
        backtracking(n, k, 1);
        return paths;
 
    }
};

216. 组合总和 III

class Solution {
public:
    // 合为n 1-9个数 k个选择
    vector<vector<int>> paths;
    vector<int> path;
    void backtracking(int n, int k, int curSum, int startIndex) {
        if(curSum>n) {
            return;
        }
 
        if(path.size()==k) {
            if(curSum==n)
                paths.push_back(path);
            return;
        }
        for(int i=startIndex; i<=9; ++i) {
            curSum += i;
            path.push_back(i);
            backtracking(n, k, curSum, i+1);
            curSum -= i;
            path.pop_back();
        }
 
    }
 
    vector<vector<int>> combinationSum3(int k, int n) {
        backtracking(n, k, 0, 1);
        return paths;
    }
};

17. 电话号码的字母组合

class Solution {
public:
    // 第startIndex个字符 总长度 digits.size() 
    vector<string> paths;
    string path;
    vector<string> myvect = {
        "",
        "",
        "abc",
        "def",
        "ghi",
        "jkl",
        "mno",
        "pqrs",
        "tuv",
        "wxyz"
    };
    
    void backtracking(string& digits, int startIndex) {
        if(path.size()==digits.size()) {
            paths.push_back(path);
            return;
        }
        int index = digits[startIndex] - '0';
        string first = myvect[index];
 
        for(int i=0; i<first.size(); ++i) {
            path.push_back(first[i]);
            backtracking(digits, startIndex+1);
            path.pop_back();
        }
 
    }
 
    vector<string> letterCombinations(string digits) {
        if(digits.size()==0) {
            return {};
        }
 
        backtracking(digits, 0);
        return paths;
    }
};