Triangular Number
Is N a triangular number — equal to 1+2+…+k for some k ≥ 1?
Input: one integer N.
Output: YES or NO.
Constraints:
- 1 ≤ N ≤ 1018
Sample Tests
Input 1
10
Output 1
YES
Input 2
11
Output 2
NO
Discussion (opens after you solve — no spoilers before)
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