**Objective :** You have been asked to Write an algorithm Find a Missing Number From a Sequence of Consecutive Numbers

**Input:** Array, arrA[] with a missing number and Range

**Output : **missing number

**Approach:**

- 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 missing number m = N-S

**Example :** suppose array given is {1,2,3,4,5,6,8,9,10} and range is 10.

So N will be sum of 1 to 10 = 10(10+1)/2 = 55

S will be sum of all the array elements which is = 48

So missing number will be = 55-48 = 7

**Complete Code:**

//find the missing number from the sequence of consecutive number | |

//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 missing number m = N-S | |

public class FindMissingNumber { | |

int Sum; | |

int Sum_N; | |

public int missingNumber(int [] arrA, int size){ | |

Sum_N = size*(size+1)/2; | |

for(int i=0;i<arrA.length;i++){ | |

Sum +=arrA[i]; | |

} | |

return Sum_N–Sum; | |

} | |

public static void main(String args[]){ | |

int [] arrA = {1,2,3,4,5,7,8,9,10}; | |

System.out.println("Missing number is :" + (new FindMissingNumber()).missingNumber(arrA,10)); | |

} | |

} |

**Output :**

Missing number is :6