HDU : 2710 (Max Factor)



// http://acm.hdu.edu.cn/showproblem.php?pid=2710
// Runtime: 15ms
// Tag: prime, sieve, factors

/* 
 * File:   main.cpp
 * Author: shahab
 * Created on March 18, 2011, 4:02 PM
 */

// @BEGIN_OF_SOURCE_CODE

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <cctype>
#include <stack>
#include <queue>
#include <list>
#include <vector>
#include <map>
#include <sstream>
#include <cmath>
#include <bitset>
#include <utility>
#include <set>
#include <numeric>

#define INF_MAX 2147483647
#define INF_MIN -2147483647
#define pi acos(-1.0)
#define N 1000000
#define LL long long

#define For(i, a, b) for ( int i = (a); i < (b); i++ )
#define Fors(i, sz) for ( size_t i = 0; i < sz.size (); i++ )
#define Set(a, s) memset (a, s, sizeof (a))

using namespace std;

bool mark [N];
vector <int> primeList;

void sieve ()
{
    memset (mark, true, sizeof (mark));

    mark [0] = mark [1] = false;

    for ( int i = 4; i < N; i += 2 ) mark [i] = false;

    for ( int i = 3; i * i <= N; i += 2 ) {
        if ( mark [i] ) {
            for ( int j = i * i; j < N; j += 2 * i ) mark [j] = false;
        }
    }

    primeList.clear ();
    primeList.push_back (2);

    for ( int i = 3; i < N; i += 2 ) {
        if ( mark [i] ) primeList.push_back (i);
    }
}

int largestPrimeFactors (int n)
{
    int ind = 0;
    int tmp = n;
    int ret = 0;

    while ( primeList [ind] * primeList [ind] <= n ) {
        while ( tmp % primeList [ind] == 0 ) {
            tmp /= primeList [ind];
            ret = primeList [ind];
        }
        ind++;
    }

    if ( tmp > 1 )
        return max (ret, tmp);
    return ret;
}


int main(int argc, char** argv) {
    sieve ();

    int n;

    while ( scanf ("%d", &n) != EOF ) {
        int maxi = INF_MIN;
        int ret;
        int num;
        while ( n-- ) {
            scanf ("%d", &num);
            int p = largestPrimeFactors(num);
            if ( maxi < p) {
                maxi = p;
                ret = num;
            }
        }
        printf ("%d\n", ret);
    }

    return 0;
}

// @END_OF_SOURCE_CODE

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