C++ program to check if number is Triangle Number.

// ? 2017 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net

#include <iostream>

using namespace std;

bool isSquare(unsigned long long n);

int main() {
  unsigned long long num;

  cout << "Enter a number: ";
  cin >> num;

  if (isSquare(8* num + 1)) {
    cout << "Number is triangle number" << endl;
    cout << "Number is not triangle number" << endl;

  int a;
  cin >> a;

  return 0;

bool isSquare(unsigned long long n) {
  for (unsigned long long i = 0; i < n / 2 + 2; i++) {
    if (i * i == n) {
      return true;

  return false;


C++ Check If Number Is Triangle Number
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