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Find two Missing Numbers in a Sequence of Consecutive Numbers

Objec­tive : Write an algo­rithm to find two Miss­ing Num­bers in a Sequence of Con­sec­u­tive Numbers

Input:  Array, arrA[] with two miss­ing num­bers and Range

Out­put : Two miss­ing numbers


    • Approach is very sim­ple, Add all the given num­bers say S
    • Cal­cu­late sum of N num­bers by for­mula n(n+1)/2 , say N
    • Find sum of two miss­ing num­bers a+b = N-S

  • Now take the prod­uct of all given num­bers say P
  • Now take the prod­uct of N num­bers , say Np;
  • Find the prod­uct of two miss­ing num­bers ab = Np–P
  • Now we have a+b and ab , we can eas­ily cal­cu­late a and b

Exam­ple :

Given array : {10,2,3,5,7,8,9,1}; Range : 10

N (Sum of 1 to 10 ) = 55

S (Sum of given elements ) = 45

a+b = 10------------------------------------(1)

Np(Product of 1 to 10) = 3628800

P(Product of given elements) = 151200

So a*b = 24---------------------------------(2)

Now we have two equations and two variables, if we solve we will get values 6 and 4.

Com­plete Code:


Missing numbers are :6 and 4


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  • kapil

    The exam­ple is very good, just a small bug at line 30 in the snip­pet, it should be (s*2) not (s*s) in the equa­tion
    diff­Sqr = (int)Math.sqrt((s*s)-4*product)