Permutation Sequence

1. Title

2. Http address

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

3. The question

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 k^th permutation sequence.

4. My code (AC)

 1     void modifyMin(char min[],int index)
 2     {
 3         for(int i = index; i < min.length - 1; i++)
 4         {
 5             min[i] = min[i+1];
 6         }
 7     }
 8     public String getPermutation(int n, int k) {
 9 
10             char str [] = new char[n];
11             char min [] = new char[n];
12             int base = 1;
13             int layer = n-1;
14             //base = (n-1)!
15             min[0] = ( char ) (1 + '0');
16             for(int i = 1 ; i <n ; i++)
17             {
18                 base *= i;
19                 min[i] = (char )(i + '1');
20             }
21             for(int i = 0 ; i < n; i++)
22             {
23                 int index = k / base;
24                 if ( k % base == 0)
25                     index --;
26             
27                 // the min permutation
28                 if( k == index * base + 1)
29                 {
30                     str[i] = min[index];
31                     modifyMin(min,index);
32                     int jj = 0;
33                     for(int j = i + 1; j < n; j++)
34                     {
35                         str[j] = min[jj++];
36                     }
37                     return new String(str);
38                 }
39 
40                 // the max permutation
41                 if ( k == (index + 1) *base ) {
42                     str[i] = min[index];
43                     modifyMin(min,index);
44                     for(int j = i + 1; j < n; j++)
45                     {
46                         str[j] = min[n-j -1];
47                     }
48 
49                     return new String(str);
50                 }
51                 
52                 str[i] = min[index];
53                 modifyMin(min,index);
54                 k -= index * base;
55             
56                 if (layer >= 2)
57                 {
58                     base /= layer;
59                     layer--;
60                 }else{
61                     base = 1;
62                 }
63             }
64             return new String(str);
65     }