Triangle(给了一系列数，判断能不能选出三个数构成三角形)

Problem Description

After Xiaoteng took a math class, he learned a lot of different shapes, but Xiaoteng's favorite triangle is because he likes stable shapes, just like his style. 

After the Xiaoxun knew it, he wanted to give a triangle as a gift to Xiaoteng. He originally planned to do one, but he was too tired. So he decided to bring some wooden sticks to Xiaoteng and let Xiaoteng make a triangle himself. 

One day, Xiaoxun brought n sticks to Xiaoteng. Xiaoteng wanted to try to use three of them to form a triangle, but the number is too much, Xiaoteng stunned, so he can only ask for your help.

 

 

Input

There are mutiple test cases.

Each case starts with a line containing a integer  n(1≤n≤5×106)  which represent the number of sticks and this is followed by n positive intergers(smaller than 231−1) separated by spaces.

Output

YES or NO for each case mean Xiaoteng can or can't use three of sticks to form a triangle.

 

Sample Input

4

1 2 3 4

Sample Output

YES

Source

2019中山大学程序设计竞赛（重现赛）

题意：输入n代表有n个数，问能不能从中选出三个构成三角形。

思路：我的思路就是：排序相邻两个数的差值很肯定最小，然后二分去找（最接近这两个数之和的那个数），比较差值和这个数的大小。

           大佬的知识：如果没有三角形，那么数列是斐波拉契数列级别增长，所以个数<50；

我的思路AC代码：

#include
#define N 5000010
#define inf 0x3f3f3f3f
#define ll long long
using namespace std;
int a[N];
void read(int &x)
{
    char s=getchar();
    while(~s&&s<'0'||s>'9')s=getchar();
    x=s-'0';
    while(~(s=getchar())&&s>='0'&&s<='9')x=x*10+s-'0';
}
int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        for(int i=0; ii&&y

运用大佬的知识AC代码：

#include
using namespace std;
#define N 5000010
int a[N];
int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        for(int i=0; i100)flag=1;
        else if(n>=3)
        {
            for(int i=2; i