代码随想录day25|216.组合总和III 17.电话号码的字母组合

216.组合总和

只能说大致套模板，自己出来了，但原理说不很清楚

class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    int sum =0;
    void backtracking(int k, int target, int sum, int index) {
        if (path.size() == k) {
            if (sum == target) result.push_back(path);
            return;
        }
        for (int i = index; i < 10 && i <=target; ++i) {
            path.push_back(i);
            sum += i;
            backtracking(k,target,sum, i+1);
            path.pop_back();
            sum -= i ;
        }
    }
    vector<vector<int>> combinationSum3(int k, int n) {
        backtracking(k, n, sum, 1);
        return result;
    }
};

17.电话号码的字母组合

加个映射，单个 char 数 转 int 最快就是减去’0’ , '0’对应ASCII 48， 'n’对应ASCII 48+n；

class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    int sum =0;
    void backtracking(int k, int target, int sum, int index) {
        if (path.size() == k) {
            if (sum == target) result.push_back(path);
            return;
        }
        for (int i = index; i < 10 && i <=target; ++i) {
            path.push_back(i);
            sum += i;
            backtracking(k,target,sum, i+1);
            path.pop_back();
            sum -= i ;
        }
    }
    vector<vector<int>> combinationSum3(int k, int n) {
        backtracking(k, n, sum, 1);
        return result;
    }
};