LeetCode: Combinations 递归回溯 2种实现方法

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题目:

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

代码：

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector<vector<int> > sumv;        if(k < 1 || k > n)            return sumv;        vector<int> tempv;        //comb1(sumv,tempv,n,k,1);        comb2(sumv,tempv,n,k,1);        return sumv;    }  /*  void comb1(vector<vector<int> >&sumv, vector<int>& tempv,int n,int k,int start)    {        for(int i= start; i <= n;i++)        {            tempv.push_back(i);            if(tempv.size() == k)            {                sumv.push_back(tempv);                tempv.pop_back();                continue;            }            if(i < n) comb(sumv,tempv,n,k,i+1);            tempv.pop_back();        }    }    */    void comb2(vector<vector<int> >&sumv, vector<int>& tempv,int n,int k, int start)    {        if(k==0)        {            sumv.push_back(tempv);            return ;        }        for(int i = start; i <= n; i++)        {            tempv.push_back(i);            comb2(sumv, tempv,n,k-1,i+1);            tempv.pop_back();        }    }};

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