Java Program to find missing numbers in an array

• To find missing numbers in an array first we need to make sure that array is sorted.
• After sorting we need to check that array each element with next element then we can find the difference.
• if Array is not sorted :To sort array use Arrays.sort(array);
• If difference is 1 then no need to do any thing because numbers are in order.
• If difference is not equal to 1 then we need to print all those numbers or pick those numbers and place it in one array.
• this would be the logic to find missing numbers in an array
• Here there may be a chance of array not starts with 1 then we need to check first itself whether array starts with 1 or not if not we need to print 1 to starting element of array.
• for example int a[]={4,5,6,8}; then we need to print 1 2 3  7.

 Lets see a java example program to find missing numbers in an array.

1. package arraysInterviewQuestions;
2. public class PrintMissingNumbers {
3.  
4. private static void findMissingNumber(int[] number){
5.  
6.         // take max length as last number in array
7.     int k[] = new int[number[number.length-1]];
8.         
9.   int m=0;
10.  
11.   if(number[0]!=1){
12.    for (int x = 1; x < number[0]; x++) {
13.        k[m] =  x;
14.        m++;
15.        }
16.   }
17.         
18.  for (int i = 0; i < number.length -1; i++) {
19.     
20.     int j = i+1;
21.     int difference = number[j] - number[i] ;
22.             
23.             
24.    if(difference != 1 ){
25.         
26.   for (int x = 1; x < difference; x++) {
27.  
28.           k[m] = number[i] + x;
29.            m++;
30.     
31. }
32.             
33.  }
34.  }
35.         
36. System.out.println("missing numbers in array ::");
37.         
38. for (int l = 0; l < m ; l++) {
39.     System.out.println( k[l]+" ");
40. }
41. }
44.  public static void main(String[] args) {
45.         
46.    int a[]= {2,4,6,9,10,20};
47.  
48.    //if Array is not sorted :To sort array use Arrays.sort(a); 
49.  
50.   findMissingNumber(a);
51.  
52.    
53. }
54. }

 

• Change the array in main method and practice try to find missing elements in an array that contains 1 to 100 numbers.