hiho 60. Permutation Sequence

問題

https://leetcode.com/problems/permutation-sequence/

解法

複雜度o(n^2)

class Solution {
public:
    string getPermutation(int n, int k) {
        bool flag[n+1];
        int f = 1;
        for (int i=1; i<=n; ++i)
        {
            f*= i;
            flag[i] = true;
        }
        string ret(n, '0');
        for (int i=0; i<n; ++i)
        {
            f /= (n-i);
            int reverse = n- i -1;
            int v;
            for (int j=reverse; j>=0; --j)
                if (j*f < k)// 第j大數 前面有j *((now-1)!) 個數， 找到最大的j
                {
                    v = j;
                    break;
                }
            k-= v*f;
            int s = -1;
            for (int j=1; j<= n; ++j)//找第v大數。
                if (flag[j])
                {
                    ++s;
                    if (s== v)
                    {
                        ret[i] += j;
                        flag[j] = false;
                        break;
                    }
                }
        }
        return ret;
    }
};