Problem
Every person shakes hands once with every other person. A handshake between A and B is the same event as one between B and A. Nobody shakes their own hand.
Implement handshake_count(people).
Input
people: The number of people.
Output
Return the total number of distinct handshakes.
Constraints
0 ≤ people ≤ 1,000,000,000. The answer fits a signed 64-bit integer.
Examples
Input:
people = 4
Expected output:
expected = 6Input:
long long people = 4;
Expected output:
long long expected = 6;Input:
long people = 4;
Expected output:
long expected = 6;Explanation
The pairs are AB, AC, AD, BC, BD, and CD: six handshakes.
Input:
people = 1
Expected output:
expected = 0Input:
long long people = 1;
Expected output:
long long expected = 0;Input:
long people = 1;
Expected output:
long expected = 0;Explanation
One person has nobody else to greet.
How to approach it
Try on your own first, then open one hint at a time.
Hint 1
Each handshake is just an unordered pair of distinct people.
Hint 2
Count how many pairs can be formed from all people without double-counting.
Hint 3
Be careful that zero or one person means no handshakes at all.
Expected complexity
O(1) time, O(1) extra space
References
This challenge is adapted for CodeUtility. Follow the statement and constraints above.