Leetcode|Combination Sum III[回溯]

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]
解法1: 4ms

class Solution {public:    vector<vector<int>> combinationSum3(int k, int n) {        vector<int> candidates={1,2,3,4,5,6,7,8,9};        vector<int> temp;        vector<vector<int>> res;        combinationSum(candidates,0,n,temp,res,k);        return res;    }    void combinationSum(vector<int>& candidates,int index,int target,vector<int> temp,vector<vector<int>> &res, int k){        if(index==candidates.size()||candidates[index]>target||temp.size()>=k) return;//终止条件        temp.push_back(candidates[index]);        if(candidates[index]==target&&temp.size()==k){            res.push_back(temp);//找到一组满足条件的             return;        }         combinationSum(candidates,index+1,target-candidates[index],temp,res,k);         temp.pop_back();         combinationSum(candidates,index+1,target,temp,res,k);    }};

解法2：0ms 加个for循环协助递归。

class Solution {public:    vector<vector<int>> combinationSum3(int k, int n) {        vector<int> candidates={1,2,3,4,5,6,7,8,9};        vector<int> temp;        vector<vector<int>> res;        combinationSum(candidates,0,n,temp,res,k);        return res;    }    void combinationSum(vector<int>& candidates,int index,int target,vector<int> temp,vector<vector<int>> &res, int k){        if(index==candidates.size()||candidates[index]>target||temp.size()>=k) return;//终止条件        for(int i=index;i<candidates.size();i++){          temp.push_back(candidates[i]);          if(candidates[i]==target&&temp.size()==k){              res.push_back(temp);//找到一组满足条件的               return;          }           combinationSum(candidates,i+1,target-candidates[i],temp,res,k);           temp.pop_back();        }           //combinationSum(candidates,index+1,target,temp,res,k);    }};