PermMissingElem

 1         /// <summary>
 2         /// Solution A
 3         /// 100/100
 4         /// </summary>
 5         /// <param name="A"></param>
 6         /// <returns></returns>
 7         public static int solution(int[] A)
 8         {
 9             HashSet<int> hs = new HashSet<int>();
10 
11             foreach (int i in A)
12             {
13                 hs.Add(i);
14             }
15 
16             for (int i = 1; i <= A.Length + 1; i++)
17             {
18                 if (!hs.Contains(i))
19                     return i;
20             }
21             return 0;
22         }
23 
24         /// <summary>
25         /// Solution B
26         /// 100/100
27         /// </summary>
28         /// <param name="A"></param>
29         /// <returns></returns>
30         public static int solution(int[] A)
31         {
32 
33             //int N = A.Length;
34             long N = A.Length;
35 
36             //总和可能超出int最大值，所以用long
37             long sumFull = (N + 1) * (N + 2) / 2;
38 
39             //long sumLack = A.Sum();
40             long sumLack = 0;
41 
42             foreach (int i in A)
43                 sumLack += i;
44 
45             return (int)(sumFull - sumLack);
46         }
47 
48         /// <summary>
49         /// Solution C
50         /// 100/100
51         /// </summary>
52         /// <param name="A"></param>
53         /// <returns></returns>
54         public static int solution(int[] A)
55         {
56             Array.Sort(A);
57             int N = A.Length;
58             for (int i = 1; i <= N; i++)
59             {
60                 if (A[i - 1] != i)
61                     return i;
62             }
63             return N + 1;
64         }