The approach used in this code is to calculate the sum of all the numbers in the input array nums, and then subtract it from the sum of all the numbers in the range [0, n]. The difference between these two sums is the missing number.
Let's go through the code step by step:
First, we declare an integer variable n and assign it the value of the size of the input vector nums. We also declare an integer variable k and initialize it to zero. Then, we loop through the input vector nums using a for loop, and for each element in the vector, we add it to the variable k. After the loop completes, k will hold the sum of all the elements in nums.
Next, we calculate the sum of all the numbers in the range [0, n] using the formula (n*(n+1))/2. We subtract the sum of nums from this value, and the result is the missing number. We assign this value to the variable k and return it from the function.
For example, let's say nums is the vector {0, 1, 3}. The size of the vector n would be 3. We would loop through the vector and calculate k as follows:
After the loop completes,k would be 4. We can then calculate the sum of all the numbers in the range [0, 3] as (3*(3+1))/2=6 . Subtracting k from this value gives us the missing number, which is 2. The function would return 2 as the output.