N - Mahmoud and a Triangle

CodeForces - 766B

Mahmoud has n line segments, the i-th of them has length ai. Ehab challenged him to use exactly 3 line segments to form a non-degenerate triangle. Mahmoud doesn't accept challenges unless he is sure he can win, so he asked you to tell him if he should accept the challenge. Given the lengths of the line segments, check if he can choose exactly 3 of them to form a non-degenerate triangle.

Mahmoud should use exactly 3 line segments, he can't concatenate two line segments or change any length. A non-degenerate triangle is a triangle with positive area.

Input
The first line contains single integer n (3 ≤ n ≤ 105) — the number of line segments Mahmoud has.

The second line contains n integers a1, a2, ..., an (1 ≤ ai ≤ 109) — the lengths of line segments Mahmoud has.

Output
In the only line print "YES" if he can choose exactly three line segments and form a non-degenerate triangle with them, and "NO" otherwise.

Example
Input
5
1 5 3 2 4
Output
YES
Input
3
4 1 2
Output
NO
Note
For the first example, he can use line segments with lengths 2, 4 and 5 to form a non-degenerate triangle.

解法：暴力穷举时间复杂度是O(n^3)显然超时，所以先排序，取相邻三条边判断

代码：

#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;

int main()
{
    int s;
    cin>>s;
    int a[100000];
    int len=0;
    for(len;len<s;len++){
        cin>>a[len];
    }
    sort(a,a+len);
    for(int i=0;i<len-2;i++)
        if(a[i]+a[i+1]>a[i+2]&&a[i]+a[i+2]>a[i+1]&&a[i+1]+a[i+2]>a[i]){
            cout<<"YES"<<endl;
            return 0;
        }
    cout<<"NO"<<endl;
    return 0;
}