C++回溯算法中组合的相关问题分析

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

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

class Solution {
private:
    vector<int> path;
    vector<vector<int>> result;
    void backtracking(vector<int>& candidates, int target,int index,int sum){
        if(sum>target) return;
        if(sum==target){
            result.push_back(path);
            return;
        }
        for(int i=index;i<candidates.size();i++){
            path.push_back(candidates[i]);
            sum+=candidates[i];
            backtracking(candidates,target,i,sum);
            sum-=candidates[i];
            path.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        backtracking(candidates,target,0,0);
        return result;
    }
};

class Solution {
private:
    vector<int> path;
    vector<vector<int>> result;
    void backtracking(vector<int>& candidates, int target,int index,int sum){
        if(sum>target) return;
        if(sum==target){
            result.push_back(path);
            return;
        }
        for(int i=index;i<candidates.size();i++){
            if(i>index&&candidates[i]==candidates[i-1])
                continue;
            path.push_back(candidates[i]);
            sum+=candidates[i];
            backtracking(candidates,target,i+1,sum);
            sum-=candidates[i];
            path.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        sort(candidates.begin(),candidates.end());
        backtracking(candidates,target,0,0);
        return result;
    }
};

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