Given a number we have to find the missing number in a given integer array which have value between 1 to 100.

**Example:**

**input**:1 2 3 4 5 6 8 9
**output**:7

**Logic:**

**step 1**- calculate the sum of array.
**step 2**-calculate the sum of first n natural number.
**step 3**-subtract both of them will give missing number.

**Code-**

#include<bits/stdc++.h> using namespace std; int main() { int arr[]={1,2,3,5,6}; int n=sizeof(arr)/sizeof(arr[0]); int m=n+1; int sum=(m*(m+1))/2; int sum1=0; for(int i=0;i<n;i++) { sum1+=arr[i]; } cout<<sum-sum1<<endl; }

**Output:**

4

**Time complexity**:0(N)

**Space Complexity**:O(1)