http://codeforces.com/problemset/problem/47/A
Triangular numbers
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output
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 test(s)
input
1
output
YES
input
2
output
NO
input
3
output
YES
AC代码：
#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int p[200000];
int main()
{
int n,i,x;
memset(p,0,sizeof(p));
for(i = 1; i <= 500; i++)
{
x = i*(i+1)/2;
p[x] = 1;
}
while(scanf("%d",&n)!=EOF)
{
if(p[n] == 1)
{
printf("YES
");
}
else
{
printf("NO
");
}
}
return 0;
}