LeetCode 60. Permutation Sequence

The set [1,2,3,…,n] contains a total of n! unique permutations.

By listing and labeling all of the permutations in order,
We get the following sequence (ie, for n = 3):

"123"
"132"
"213"
"231"
"312"
"321"

Given n and k, return the kth permutation sequence.

Note: Given n will be between 1 and 9 inclusive.

cantor展开

直接用cantor展开就可以了。

关于cantor展开，请参考：http://www.cnblogs.com/aiterator/p/6675416.html

class Solution
{
public:
    string getPermutation(int n, int k)
    {
        k --;
        int res = 1;
        list<int> lst;
        for(int i=1; i<=n; ++ i)
        {
            lst.push_back(i);
            res *= i;
        }

        string str;
        while(n)
        {
            res /= n --;

            int t = k / res;
            auto it = lst.begin();
            while(t --)
                it ++;
            str += *it + '0';
            lst.erase(it);

            k %= res;
        }
        return str;
    }
};