Problem
Find the missing number from the list of 99 distinct numbers which are from 1-100
Solution
Compute sum of the list.
Compute desirable sum without missing number.
The missing number is desirable sum minus actual sum.
[csharp]
using System;
using System.Collections.Generic;
using System.Linq;
namespace MissingNumber
{
class Program
{
static int FindMissingNumber(List<int> list, int low, int up)
{
int sum = (up + low) * (up - low + 1) >> 1;
Console.WriteLine("Expected sum without missing {0}", sum);
Console.WriteLine("Actual sum without missing {0}", list.Sum());
return sum - list.Sum();
}
static void Main(string[] args)
{
List<int> list = new List<int>();
Random random = new Random(100);
int low = 1;
int up = 100;
int missNumber = random.Next(low, up);
for (int i = low; i < missNumber; ++i)
{
list.Add(i);
}
for (int i = missNumber + 1; i <= up; ++i)
{
list.Add(i);
}
int line = 0;
foreach (int k in list)
{
Console.Write("{0, 4} ", k);
line++;
if (line == 10)
{
line = 0;
Console.WriteLine();
}
}
Console.WriteLine();
Console.WriteLine("The missing number is: {0}.", FindMissingNumber(list, low, up));
}
}
}
using System;
using System.Collections.Generic;
using System.Linq;
namespace MissingNumber
{
class Program
{
static int FindMissingNumber(List<int> list, int low, int up)
{
int sum = (up + low) * (up - low + 1) >> 1;
Console.WriteLine("Expected sum without missing {0}", sum);
Console.WriteLine("Actual sum without missing {0}", list.Sum());
return sum - list.Sum();
}
static void Main(string[] args)
{
List<int> list = new List<int>();
Random random = new Random(100);
int low = 1;
int up = 100;
int missNumber = random.Next(low, up);
for (int i = low; i < missNumber; ++i)
{
list.Add(i);
}
for (int i = missNumber + 1; i <= up; ++i)
{
list.Add(i);
}
int line = 0;
foreach (int k in list)
{
Console.Write("{0, 4} ", k);
line++;
if (line == 10)
{
line = 0;
Console.WriteLine();
}
}
Console.WriteLine();
Console.WriteLine("The missing number is: {0}.", FindMissingNumber(list, low, up));
}
}
}
Output
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50
51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80
81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 97 98 99 100
Expected sum without missing 5050
Actual sum without missing 4954
The missing number is: 96.
