UVa: 10131 - Is Bigger Smarter?

#include <algorithm>
#include <iostream>
#include <vector>
#include <stack>
#define x first
#define y second

using namespace std;

int main()
{
    vector <pair<pair<int, int>, int> > a;

    int u, v, i=0;
    while(cin >> u >> v)
        a.push_back(make_pair(make_pair(u, v), i)), i++;

    sort(a.begin(), a.end());

    int n = a.size();

    vector <int> lis(n, 1);
    vector <int> from(n, -1);

    int p = 0; ///Posición final de LIS
    int m = 1; ///LIS
    for(int i=1; i<n; i++)
    {
        for(int j=0; j<i; j++)
        {
            if(a[j].x.y > a[i].x.y && lis[i] < lis[j]+1)
            {
                lis[i] = lis[j]+1;
                from[i] = j;
            }
        }
        if(lis[i] > m)
        {
            m = lis[i];
            p = i;
        }
    }

    stack<int> r;

    r.push(p);
    while(from[p] != -1)
    {
        r.push(from[p]);
        p = from[p];
    }

    cout << r.size() << endl;
    while(r.size())
    {
        int z = r.top();
        r.pop();
        cout << a[z].y+1 << endl;
    }
    return 0;
}