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Description
A triangular number is the number of dots in an equilateral triangle uniformly filled with dots. For example, three dots can be arranged in a triangle; thus three is a triangular number. The n-th triangular number is the number of dots in a triangle with n dots on a side. 
. You can learn more about these numbers from Wikipedia (http://en.wikipedia.org/wiki/Triangular_number).
Your task is to find out if a given integer is a triangular number.
Input
The first line contains the single number n (1 ≤ n ≤ 500) — the given integer.
Output
If the given integer is a triangular number output YES, otherwise output NO.
Sample Input
1
YES
2
NO
3
YES
#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<queue>
#include<vector>
#include<set>
using namespace std;
int t,n;
int main()
{
while(scanf("%d",&t)!=EOF)
{
n=(int)sqrt(2*t);
if(n*(n+1)==2*t)
printf("YES\n");
else
printf("NO\n");
}
return 0;
}
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原文地址：http://www.cnblogs.com/a972290869/p/4215566.html
