47A - Triangular numbers - CodeForces Solution

brute force math *800

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Python Code: 

import math
n=int(input())
if pow(int(math.sqrt(1+(8*n))),2)==1+(8*n):
    print("YES")
else:
    print("NO")

C++ Code: 

#include <bits/stdc++.h>
using namespace std;

int main() {
  int n;
  cin >> n;
  int i = 1;
  while (i * (i + 1) < 2 * n) {
    i++;
  }
  if (i * (i + 1) == 2 * n) {
    cout << "YES";
  } else {
    cout << "NO";
  }
}


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