标签：
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], ]
class Solution {
public:
vector<vector<int>> res;
vector<int> part;
void com(int dep,int maxdep,int n,int start)
{
if(dep==maxdep)
{
res.push_back(part);
return;
}
for(int i=start;i<=n;i++)
{
part[dep]=i;
com(dep+1,maxdep,n,i+1);
}
}
vector<vector<int>> combine(int n, int k) {
if(n<1) return res;
part.resize(k);
if(k==0||k==n)
{
part.resize(n);
for(int i=1;i<=n;i++)
part[i-1]=i;
res.push_back(part);
return res;
}
com(0,k,n,1);
return res;
}
};class Solution {
public:
int int_to_bool(int n,vector<int>&index)//返回1出现的次数，也就是组合中元素的个数
{
int len=index.size();
index.clear();
index.resize(len);
int i=0;
int add=0;
while(n)
{
int temp=n%2;
index[i]=temp;
if(temp==1)
add++;
i++;
n/=2;
}
return add;
}
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> res;
if(n<1) return res;
vector<int> num;
for(int i=1;i<=n;i++)
num.push_back(i);
if(k==0||k==n)
{
res.push_back(num);
return res;
}
vector<int> index(n,0);
for(int i=0;i<(1<<n);i++)
{
if(int_to_bool(i,index)==k)
{
vector<int> temp;
for(int j=0;j<n;j++)
{
if(index[j]==1)
temp.push_back(num[j]);
}
res.push_back(temp);
}
}
return res;
}
};
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原文地址：http://blog.csdn.net/sinat_24520925/article/details/46835569
