#include <bits/stdc++.h>
using namespace std;
int main() {
    int a[3];
    cin >> a[0] >> a[1] >> a[2];
    sort(a,a+3);
    if(a[0]+a[1]<=a[2]) {
        cout << "Not triangle" << endl;
        return 0;
    }
    if(a[0]*a[0]+a[1]*a[1]==a[2]*a[2]) {
        cout << "Right triangle" << endl;
    } else if(a[0]*a[0]+a[1]*a[1]>a[2]*a[2]) {
        cout << "Acute triangle" << endl;   //A cute triangle =w=
    } else {
        cout << "Obtuse triangle" << endl;
    }
    if(a[0]==a[1]) {
        if(a[1]==a[2]) {
            cout << "Isosceles triangle" << endl;
            cout << "Equilateral triangle" << endl;
        } else {
            cout << "Isosceles triangle" << endl;
        }
    } else {
        if(a[1]==a[2]) {
            cout << "Isosceles triangle" << endl;
        }
    }
    return 0;
}

对输入的数字从小到大排序以避免啰嗦的判断写法，注意 $a+b==c$ 时也是无法组成三角形的

#include <bits/stdc++.h>
using namespace std;
int main() {
    int a[3];
    cin >> a[0] >> a[1] >> a[2];
    sort(a,a+3);
    if(a[0]+a[1]<=a[2]) {
        cout << "Not triangle" << endl;
        return 0;
    }
    if(a[0]*a[0]+a[1]*a[1]==a[2]*a[2]) {
        cout << "Right triangle" << endl;
    } else if(a[0]*a[0]+a[1]*a[1]>a[2]*a[2]) {
        cout << "Acute triangle" << endl;   //A cute triangle =w=
    } else {
        cout << "Obtuse triangle" << endl;
    }
    if(a[0]==a[1]) {
        if(a[1]==a[2]) {
            cout << "Isosceles triangle" << endl;
            cout << "Equilateral triangle" << endl;
        } else {
            cout << "Isosceles triangle" << endl;
        }
    } else {
        if(a[1]==a[2]) {
            cout << "Isosceles triangle" << endl;
        }
    }
    return 0;
}

检查了一下我之前写的代码，发现它现在不能通过新的测试点 #6，因为它其实是有缺陷的：当 $a[0]\neq a[1]$ 但 $a[1] == a[2]$ 时它只会输出一行 Acute triangle 而不会输出 Isosceles triangle，例如 2 5 5, 5 2 5, 5 5 2 这样的输入很明显是等腰三角形。

#include <bits/stdc++.h>
using namespace std;
int a[3];
int main()
{
	scanf("%d%d%d", &a[0], &a[1], &a[2]);
	sort(a, a + 3);
	if((a[0] + a[1]) < a[2])
	{
		printf("Not triangle\n");
		return 0;
	}
	if((a[0] * a[0]) + (a[1] * a[1]) == (a[2] * a[2]))
	  printf("Right triangle\n");
	if((a[0] * a[0]) + (a[1] * a[1]) > (a[2] * a[2]))
	  printf("Acute triangle\n");
	if((a[0] * a[0]) + (a[1] * a[1]) < (a[2] * a[2]))
	  printf("Obtuse triangle\n");
	if(a[0] == a[1])
	  if(a[1] == a[2])
	  {
	  	printf("Isosceles triangle\n");
	    printf("Equilateral triangle\n");
	  }
	  else
	  	printf("Isosceles triangle\n");
	return 0;
}