《挑战程序设计竞赛(第2版)》 1.6.1 三角形 � 码农场


作为《挑战程序设计竞赛(第2版)》第一章最开始的“简单题”,直接三重循环遍历你就输了。给出一个O(nlogn)的算法,先排序O(nlogn),然后遍历至多n – 2次得出结果:
有n根棍子,棍子i的长度为ai,想要从中选出3根棍子组成周长尽可能长的三角形,请输出最大的周长,若无法组成三角形则输出0。
       例如:
       输入:
              n= 5
              a= { 2, 3, 4, 5, 10}
       输出:
              12(选择3,4,5时)
int main(int argc, char *argv[])
{
#ifndef ONLINE_JUDGE
    freopen("in.txt""r", stdin);
    freopen("out.txt""w", stdout);
#endif
    int n;
    cin >> n;
    vector<int> coll;
    copy(istream_iterator<int>(cin), istream_iterator<int>(), back_inserter(coll));
    sort(coll.begin(), coll.end(), greater<int>());
    //copy(coll.begin(), coll.end(), ostream_iterator<int>(cout, " "));
    bool found = false;
    for (int i = 0; i < n - 2; ++i)
    {
        if (coll[i] < coll[i + 1] + coll[i + 2])
        {
            cout << coll[i] + coll[i + 1] + coll[i + 2] << endl;
            found = true;
            break;
        }
    }
    if (!found)
    {
        cout << 0 << endl;
    }
#ifndef ONLINE_JUDGE
    fclose(stdin);
    fclose(stdout);
    system("out.txt");
#endif
    return 0;
}
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