参考答案：

#include<iostream>
#include<algorithm>
using namespace std;
const int N = 20;
struct node
{
    long long a;
    long long b;
    long long c;
};
bool cmp(struct node a1, struct node b1)
{
    return a1.c < b1.c;
}
int main()
{
    int n;
    cin >> n;
    struct node a[N];
    for (int i = 1; i <= n; ++i)
    {
        cin >> a[i].a >> a[i].b;
        a[i].c = a[i].a * a[i].a + a[i].b * a[i].b;
    }
    sort(a + 1, a + 1 + n, cmp);
    for (int i = 1; i <= n; ++i)
    {
        cout << a[i].a << ' ' << a[i].b << ' ' << a[i].c << endl;
    }

    return 0;
}

Test lambda 表达式用于排序

#include<bits/stdc++.h>
#define int long long
#define PII pair<int,int>
#define ULL unsigned long long
#define all(v) v.begin(), v.end()
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
constexpr int N =  1 * 1e6 + 10,M = 5 * 1e3 + 10,inf = 0x3f3f3f3f;

struct node
{
    int x,y,dis;
};
void solve()
{
    int n;
    cin >> n;
    vector<node> vec(n);
    for(int i=0;i<n;i++)
    {
        auto &[x,y,dis] = vec[i];
        cin >> x >> y;
        dis = x*x+y*y;
    }

    sort(all(vec),[](const node &a,const node &b){
        return a.dis < b.dis;
    });

    for(auto [x,y,dis] : vec) cout << x << ' ' << y << ' ' << dis << '\n';
}
signed main()
{
    ios::sync_with_stdio(0);cin.tie(nullptr),cout.tie(nullptr);
    int _=1;
    // cin>>_;
    while(_--)
    {
        solve();
    }
    return 0;
}