LeetCode: Permutation Sequence 解题报告

 1 package Algorithms.permutation;
 2 
 3 /*
 4  The set [1,2,3,…,n] contains a total of n! unique permutations.
 5 
 6 By listing and labeling all of the permutations in order,
 7 We get the following sequence (ie, for n = 3):
 8 
 9     "123"
10     "132"
11     "213"
12     "231"
13     "312"
14     "321"
15 
16 Given n and k, return the kth permutation sequence.
17 
18 Note: Given n will be between 1 and 9 inclusive.
19  * */
20 public class PermutationSequence {
21     public static String getPermutation(int n, int k) {
22         if (n == 0) {
23             return "";
24         }
25         
26         // 先计算出(n)!
27         int num = 1;
28         for (int i = 1; i <= n; i++) {
29             num *= i;
30         }
31         
32         boolean[] use = new boolean[n];
33         for (int i = 0; i < n; i++) {
34             use[i] = false;
35         }
36         
37         // 因为index是从0开始计算 
38         k--;
39         StringBuilder sb = new StringBuilder();
40         for (int i = 0; i < n; i++) {
41             // 计算完第一个数字前，num要除以(n)
42             num = num / (n - i);
43             
44             int index = k / num;
45             k = k % num;
46             
47             for (int j = 0; j < n; j++) {                
48                 if (!use[j]) {
49                     if (index == 0) {
50                         // 记录下本次的结果.
51                         sb.append((j + 1) + "");
52                         use[j] = true;
53                         break;
54                     }
55                     
56                     // 遇到未使用过的数字，记录index
57                     index--;
58                 }
59             }
60         }
61         
62         return sb.toString();
63     }
64 
65     public static void main(String[] args) {
66         System.out.println(getPermutation(3, 5));
67     }
68 
69 }

 1 public String getPermutation1(int n, int k) {
 2         // 1:17 -> 1:43
 3         LinkedList<Character> digits = new LinkedList<Character>();
 4         
 5         // bug 2: should only add n elements.
 6         for (char i = ‘1‘; i <= ‘0‘ + n; i++) {
 7             digits.add(i);
 8         }
 9         
10         k = k - 1;
11         StringBuilder sb = new StringBuilder();
12         
13         int sum = 1;
14         // n!
15         for (int i = 1; i <= n; i++) {
16             sum *= i;
17         }
18         
19         int cur = n;
20         while (!digits.isEmpty()) {
21             sum /= cur;
22             cur--;
23             
24             int digitIndex = k / sum;
25             k = k % sum;
26             //Line 25: error: cannot find symbol: method digits(int)
27             sb.append(digits.get(digitIndex));
28             // remove the used digit.
29             digits.remove(digitIndex);
30         }
31         
32         return sb.toString();
33     }

 1 public String getPermutation(int n, int k) {
 2         // 1:17 -> 1:43
 3         LinkedList<Character> digits = new LinkedList<Character>();
 4         
 5         // bug 2: should only add n elements.
 6         for (char i = ‘1‘; i <= ‘0‘ + n; i++) {
 7             digits.add(i);
 8         }
 9         
10         // The index start from 0;
11         k--;
12         StringBuilder sb = new StringBuilder();
13         
14         int sum = 1;
15         // n!
16         for (int i = 1; i <= n; i++) {
17             sum *= i;
18         }
19         
20         for (int i = n; i >= 1; i--) {
21             sum /= i;
22             int digitIndex = k / sum;
23             k = k % sum;
24             
25             //Line 25: error: cannot find symbol: method digits(int)
26             sb.append(digits.get(digitIndex));
27             
28             // remove the used digit.
29             digits.remove(digitIndex);
30         }
31         
32         return sb.toString();
33     }