Count ways to express a number as sum of exactly two numbers
Given a positive integer N. The task is to find the number of ways in which you can express N as a sum of exactly two numbers A and B (N = A + B) where A > 0, B > 0 and B > A.
Input: N = 8 Output: 3 Explanation: N = 8 can be expressed as (1, 7), (2, 6), (3, 5) Input: N = 14 Output: 6
- An observation here is that for every number N, if we take a number A which is less than N/2, then there must be a number B which is greater than N/2 and A + B = N.
- This leads to a simple solution of finding the count of numbers for either B or A. Hence, the floor value of (N-1)/2 will lead to the solution.
Time complexity: O(1)
Auxiliary space: O(1) because it is using constant space for variable
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