立志重刷代码随想录60天冲冲冲！！——第二十二天

第77题. 组合

class Solution {
public:
    vector<vector<int>> res;
    vector<int> path;
    void backtracking(int n, int k, int startIdx) {
        if (path.size() == k) {
            res.push_back(path);
            return;
        }

        for (int i = startIdx; i <= n; i++) {
            path.push_back(i);
            backtracking(n, k, ++startIdx);
            path.pop_back();
        }
        return;
    }
    vector<vector<int>> combine(int n, int k) {
        backtracking(n, k, 1);
        return res;
    }
};

216.组合总和III

class Solution {
public:
    int sum = 0;
    vector<int> path;
    vector<vector<int>> res;
    void backtracking (int k, int n, int startIdx) {
        if (path.size() == k && sum == n) {
            res.push_back(path);
            return;
        }

        for (int i = startIdx; i <= 9; i++) {
            path.push_back(i);
            sum += i;
            backtracking(k, n, ++startIdx);
            sum -= i;
            path.pop_back();
        }
        return;
    }
    vector<vector<int>> combinationSum3(int k, int n) {
        backtracking(k, n, 1);
        return res;
    }
};

17.电话号码的字母组合

之前一直有问题，我是用了++index，不能用自增，会越界

class Solution {
public:
    string umap[10] = {
        "",
        "",
        "abc",
        "def",
        "ghi",
        "jkl",
        "mno",
        "pqrs",
        "tuv",
        "wxyz"
    };
    string path;
    vector<string>res;
    void backtracking(string digits, int index) {
        if (digits.size() == index) {
            res.push_back(path);
            return;
        }

        int digit = digits[index] - '0';
        string letters = umap[digit];
        for (int i = 0; i < letters.size(); i++) {
            path.push_back(letters[i]);
            backtracking(digits, index + 1);
            path.pop_back();
        }
        return;
    }
    vector<string> letterCombinations(string digits) {
        if (digits == "") return res;
        backtracking(digits, 0);
        return res;
    }
};