代码随想录Day22

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LeetCode77.组合

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

LeetCode216.组合总和Ⅲ

class Solution {
public:
    vector<vector<int>> result;
    vector<int> combine;
    int sum = 0;
    vector<vector<int>> combinationSum3(int k, int n) {
        dfs(k, n, 1, sum);
        return result;
    }
    void dfs(int k, int n, int startIndex, int sum){
        if(combine.size() == k){
            if(sum == n){
                result.push_back(combine);
            }
            return;
        }
        for(int i = startIndex; i <= 9 - (k - combine.size()) + 1; i++){
            combine.push_back(i);
            sum += i;
            dfs(k, n, i + 1, sum);
            sum -= i;
            combine.pop_back();
        }
    }
};

LeetCode17.电话号码的字母组合

class Solution {
public:
    std::map<char,std::vector<char>> numToLetter = {{'2',{'a','b','c'}},{'3',{'d','e','f'}},{'4',{'g','h','i'}},{'5',{'j','k','l'}},
    {'6',{'m','n','o'}},{'7',{'p','q','r','s'}},{'8',{'t','u','v'}},{'9',{'w','x','y','z'}}};
    std::string combine = "";
    vector<string> result;
    vector<string> letterCombinations(string digits) {
        if(digits == ""){
            return result;
        }
        dfs(digits,0);
        return result;
    }
    void dfs(string digits, int index){
        if(combine.length() == digits.length()){
            result.push_back(combine);
            return;
        }
        char cur = digits[index];
        for(int i = 0; i < numToLetter[cur].size(); i++){
            combine.push_back(numToLetter[cur][i]);
            dfs(digits,index + 1);
            combine.pop_back();
        }
    }
};