Permutations
Given a collection of numbers, return all possible permutations.
For example,[1,2,3] have the following permutations:[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].
解法一：递归
class Solution {
public:
vector<vector<int> > permute(vector<int> &num) {
vector<vector<int> > ret;
Helper(ret, num, );
return ret;
}
void Helper(vector<vector<int> >& ret, vector<int> num, int pos)
{
if(pos == num.size()-)
ret.push_back(num);
else
{
for(int i = pos; i < num.size(); i ++)
{//swap all the ints to the current position
swap(num[pos], num[i]);
Helper(ret, num, pos+);
swap(num[pos], num[i]);
}
}
}
};解法二：just a joke
别忘了先排序，因为next_permutation是升序返回的。
class Solution
{
public:
vector<vector<int> > permute(vector<int> &num)
{
vector<vector<int> > result;
//sort first
//note that next_permutation is in ascending order
sort(num.begin(), num.end());
result.push_back(num);
while(next_permutation(num.begin(), num.end()))
{
result.push_back(num);
}
return result;
}
};