代码随想录day22

77.组合

//理解回溯的原理及剪枝操作

    vector<vector<int>> result;
    vector<int> path;
    void backtracking(int n, int k, int start){
        if(path.size() == k){
            result.push_back(path);
            return;
        }
        for(int i = start; i <= n-(k-path.size())+1; i++){
            path.push_back(i);
            backtracking(n, k, i+1);
            path.pop_back();
        }
    }
    vector<vector<int>> combine(int n, int k) {
        result.clear();
        path.clear();
        backtracking(n,k,1);
        return result;
    }

216.组合总和III

    vector<vector<int>> result;
    vector<int> path;
    void backtrace(int n, int k, int startIndex){
        if(path.size() == k){
            if(n == 0){
                result.push_back(path);
            }
            return;
        }
        for(int j = startIndex; j <= 9; j++){
            path.push_back(j);
            n -= j;
            backtrace(n,k,j+1);
            n += j;
            path.pop_back();
        }
    }
    vector<vector<int>> combinationSum3(int k, int n) {
        result.clear();
        path.clear();
        backtrace(n, k, 1);
        return result;
    }

17.电话号码的字母组合，需二刷

    const string letterMap[10] = {
        "",
        "",
        "abc",
        "def",
        "ghi",
        "jkl",
        "mno",
        "pqrs",
        "tuv",
        "wxyz"
    };
public:
    vector<string> result;
    string s;
    void backtracking(const string & digits, int start){
        if(start == digits.size()){
            result.push_back(s);
            return;
        }
        int digit = digits[start] - '0';
        string letter = letterMap[digit];
        for(int i = 0; i < letter.size(); i++){
            s.push_back(letter[i]);
            backtracking(digits, start+1);
            s.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        result.clear();
        s.clear();
        if(digits.size() == 0){
            return result;
        }
        backtracking(digits, 0);
        return result;
    }