Triangular numbers
time limit per test2 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard 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

用一个哈希表存储哪些是triangular number，满足这个数字的要求就是这个数能由公式得到。
AC代码：

#include
#include
#include

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\n");
        }
        else
        {
            printf("NO\n");
        }
    }

    return 0;
}