216.leetcode Combination Sum III(medium)[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.

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]]

题目的意思是：求用k个[1,9]的数字组成和为n的数组集合，其中每个数字不能重复，对于这种遍历有限制条件的问题可以采用回溯的方式完成。

class Solution {public:  void combination(vector<int> &temp,vector<vector<int> > &result,int sum,int k,int n,int i)    {        if(temp.size() == k)        {            if(sum == n)            {                result.push_back(temp);            }            return;        }            for(int j=i;j<=9;j++)            {                temp.push_back(j);                combination(temp,result,sum+j,k,n,j+1);                temp.erase(temp.end()-1);//没满足条件的回溯了            }    }    vector<vector<int>> combinationSum3(int k, int n) {        //采用回溯法来完成        vector<vector<int> > result;        vector<int> temp;        if(k<=0 || n<=0 || n<k || n>9*k) return result;        if(k == 1)//只要一个数，特殊处理        {            temp.push_back(n);            result.push_back(temp);            return result;        }        combination(temp,result,0,k,n,1);        return result;    }};