문제
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.
제약 조건
0 ≤ people ≤ 1,000,000,000. The answer fits a signed 64-bit integer.
예시
예시 1
입력:
people = 4
예상 출력:
expected = 6입력:
long long people = 4;
예상 출력:
long long expected = 6;입력:
long people = 4;
예상 출력:
long expected = 6;설명
The pairs are AB, AC, AD, BC, BD, and CD: six handshakes.
예시 2
입력:
people = 1
예상 출력:
expected = 0입력:
long long people = 1;
예상 출력:
long long expected = 0;입력:
long people = 1;
예상 출력:
long expected = 0;설명
One person has nobody else to greet.
접근 방법
먼저 스스로 풀어 본 뒤 힌트를 하나씩 열어 보세요.
힌트 1
Each handshake is just an unordered pair of distinct people.
힌트 2
Count how many pairs can be formed from all people without double-counting.
힌트 3
Be careful that zero or one person means no handshakes at all.
예상 복잡도
O(1) time, O(1) extra space
참고 자료
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