Problem:
Score : 500 points

Problem Statement

Given are N integers A_1,\ldots,A_N.

We will choose exactly K of these elements. Find the maximum possible product of the chosen elements.

Then, print the maximum product modulo (10^9+7), using an integer between 0 and 10^9+6 (inclusive).

Constraints

1 \leq K \leq N \leq 2\times 10^5

|A_i| \leq 10^9

Input

Input is given from Standard Input in the following format:

N K
A_1 \ldots A_N

Output

Print the maximum product modulo (10^9+7), using an integer between 0 and 10^9+6 (inclusive).

Sample Input 1

4 2
1 2 -3 -4

Sample Output 1

12

The possible products of the two chosen elements are 2, -3, -4, -6, -8, and 12, so the maximum product is 12.

Sample Input 2

4 3
-1 -2 -3 -4

Sample Output 2

1000000001

The possible products of the three chosen elements are -24, -12, -8, and -6, so the maximum product is -6.

We print this value modulo (10^9+7), that is, 1000000001.

Sample Input 3

2 1
-1 1000000000

Sample Output 3

1000000000

The possible products of the one chosen element are -1 and 1000000000, so the maximum product is 1000000000.

Sample Input 4

10 10
1000000000 100000000 10000000 1000000 100000 10000 1000 100 10 1

Sample Output 4

999983200

Be sure to print the product modulo (10^9+7).

Solution in Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)

    import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedList;
import java.util.StringTokenizer;
import java.util.TreeSet;

public class Main {
	static ArrayList<Integer> adj[];
	//static PrintWriter out = new PrintWriter(System.out);

	static int [][]notmemo;
	static int k;
	static int[] a;
	static int b[];
	static int m;


	static class Pair  implements Comparable<Pair>{
    	int u ;int idx;
		public Pair(int a,int a2) {
		u=a;
	   idx=a2;
		}
		@Override
		public int compareTo(Pair o) {
			if(this.u==o.u)
				return this.idx-o.idx;
			else
				return this.u-o.u;
		}
		
    }
    static Pair s1[];
    static int s;
    static ArrayList<Pair> adjlist[];
	static char c[];
	public static long mod= 1000000007;

	public static void main(String args[]) throws Exception {
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		int n=sc.nextInt();
		int k=sc.nextInt();
		long a[]=new long[n];
		for (int i = 0; i < a.length; i++) {
			a[i]=sc.nextInt();
		}
		ArrayList<Long> pos=new ArrayList<>();
		ArrayList<Long> neg=new ArrayList<>();
		for (int i = 0; i < a.length; i++) {
			if(a[i]>=0) {
				pos.add(a[i]);
			}
			else
				neg.add(a[i]);
			
		}
		Collections.sort(pos);
		Collections.sort(neg);
		Collections.reverse(pos);
		boolean fix=false;
		if(pos.size()==0&&k%2!=0) {
			Collections.reverse(neg);
		}
		else  {
			fix=true; 
		}
		
		
		int p1=0;
		int p2=0;
		ArrayList<Long> ans=new ArrayList<>();
		for (int i = 0; i <k; i++) {
		if(p1+1<pos.size()&&p2+1<neg.size()&&i+1<k) {
			long trial1=1l*pos.get(p1)*1l*pos.get(p1+1)*1l;
			long trial2=1l*neg.get(p2)*1l*neg.get(p2+1)*1l;
			if(trial1>=trial2) {
				ans.add(pos.get(p1));
				ans.add(pos.get(p1+1));
				p1+=2;
				i++;
			}
			else {
				ans.add(neg.get(p2));
				ans.add(neg.get(p2+1));
				p2+=2;
				i++;
			}
		}
		else if(p1<pos.size()&&p2<neg.size()) {
			long mx;
			ans.add(mx=Math.max(pos.get(p1),-neg.get(p2)));
			if(mx==pos.get(p1))
				p1++;
				else
					p2++;
		}
		else if(p1<pos.size()){
			ans.add(pos.get(p1));
			p1++;
		}
		else {
			ans.add(neg.get(p2));
			p2++;
			
		}
		}
		Collections.sort(ans);
		if(fix) {
			int countneg=0;
			long minpos=(int) (1e9+1);
			int p=0;
			int pp=0;
			for(long x:ans) {
				if(x<0)
					countneg++;
				else {
				if(minpos>x) {
					minpos=x;
				pp=p;
				}
				}
				p++;
				
			}
			if(countneg%2!=0) {
				if(p1==pos.size()&&p2==neg.size()) {
					
				}
				else if(p2==neg.size()) {
					ans.remove(0);
					ans.add(pos.get(p1));
				}
				else if(p1==pos.size()) {
					ans.remove(pp);
					ans.add(neg.get(p2));
				}
				else {
					minpos*=pos.get(p1);
					long lol=ans.get(0)*neg.get(p2);
					if(lol>=minpos) {
						ans.add(neg.get(p2));
						ans.remove(pp);
					}
					else{
						ans.remove(0);
						ans.add(pos.get(p1));
					}
				}
			}
		}
		long total=1; 
		for(long x:ans) {
			
			total*=((x+mod)%mod);
			total=(total+mod)%mod;
		}
		System.out.println((total+mod)%mod);
	}
	
	private static boolean works(int mid) {
	label:for (int i = 0; i < a.length; i++) {
		int size=(b[i]-a[i])+1;
		if(size<mid) {
			continue;
		}
		if(i+mid>a.length)
			break;
		for (int j = i+1; j <i+mid&&j<a.length; j++) {
			
			if(b[i]<=a[j]||b[j]<=a[i]) {
				continue label;	
			}
			if(a[i]>=a[j]&&b[i]>=b[j]) {
				int diff=(b[j]-a[i])+1;
			if(diff<mid) {
				continue label;
			}
			
			}
			if(a[i]<=a[j]&&b[i]>=b[j]) {
				int diff=(b[j]-a[j])+1;
			if(diff<mid) {
				continue label;
			}
			
			}
			else if(a[i]>=a[j]&&b[i]<=b[j]) {
				int diff=(b[i]-a[j])+1;
			if(diff<mid) {
				continue label;
			}
			
			}
			else if(a[i]<=a[j]&&b[i]<=b[j]) {
				int diff=(b[i]-a[j])+1;
			if(diff<mid) {
				continue label;
			}
			
			}
			
			
			
		}
		return true;
	}
	return false;
	}

	static int[][] memo;


	static int dp(int idx,int state) {
		if(idx==n) {
			return 0;
		}
		int take=0;
		int leave=dp(idx+1,state);
		if(memo[idx][state]!=-1) {
			return memo[idx][state];
		}
		if(c[idx]=='3'&&state==1) {
			take=dp(idx+1,state);
		}
		else if(c[idx]=='3'&&state!=1) {
			take=1+dp(idx+1,2);
		}
		else if(c[idx]=='1') {
			take=1+dp(idx+1,1);
		}
		
		return memo[idx][state]=Math.max(take,leave);
		
		
		
		
	}
	static long modmod=998244353;
	static int dx[]= {1,-1,0,0};
	 static int dy[]= {0,0,1,-1};

	
	static class BBOOK implements Comparable<BBOOK>{
		int t;
		int alice;
		int bob;
		public BBOOK(int x,int y,int z) {
		t=x;
		alice=y;
		bob=z;
		}
		@Override
		public int compareTo(BBOOK o) {
			return this.t-o.t;
		}
	}
	static boolean longvis[][];

	static int reali,realj;
	 static char curchar;
	
	



	private static int lcm(int a2, int b2) {
	return (a2*b2)/gcd(a2,b2);
	}
	static boolean visit[][];

	static Boolean zero[][];
	static int small[];
	
		
		
		
	
	static int idx=0;
	
	static long dpdp[][][][];
	static int modd=(int) 1e8;
	
		


	static class Edge implements Comparable<Edge>
	{
		int node;long cost;
		
		Edge(int a, long b) { node = a;	cost = b; }
		
		public int compareTo(Edge e){ return Long.compare(cost,e.cost);	}
	}
	
        static void sieve(int N)	// O(N log log N) 
	{
		isComposite = new int[N+1];					
		isComposite[0] = isComposite[1] = 1;			// 0 indicates a prime number
		primes = new ArrayList<Integer>();
 
		for (int i = 2; i <= N; ++i) 					//can loop till i*i <= N if primes array is not needed O(N log log sqrt(N)) 
			if (isComposite[i] == 0) 					//can loop in 2 and odd integers for slightly better performance
			{
				primes.add(i);
				if(1l * i * i <= N)
					for (int j = i * i; j <= N; j += i)	// j = i * 2 will not affect performance too much, may alter in modified sieve
						isComposite[j] = 1;
			}   
	}
    static TreeSet<Integer> factors;
    static ArrayList<Integer> primeFactors(int N)		// O(sqrt(N) / ln sqrt(N))
	{
		ArrayList<Integer> factors = new ArrayList<Integer>();		//take abs(N) in case of -ve integers
		int idx = 0, p = primes.get(idx);

		while(1l*p * p <= N)
		{
			while(N % p == 0) { factors.add(p); N /= p; }
			p = primes.get(++idx);
		}

		if(N != 1)						// last prime factor may be > sqrt(N)
			factors.add(N);				// for integers whose largest prime factor has a power of 1
		return factors;
	}
    
    static String y;
    	static int nomnom[];
	static long fac[];
	



	static boolean f = true;

	static class SegmentTree { // 1-based DS, OOP

		int N; 			//the number of elements in the array as a power of 2 (i.e. after padding)
		int[] array, sTree, lazy;
		
		SegmentTree(int[] in)		
		{
			array = in; N = in.length - 1;
			sTree = new int[N<<1];		//no. of nodes = 2*N - 1, we add one to cross out index zero
			lazy = new int[N<<1];
			Arrays.fill(sTree, (int) 1e9);
			build(1,1,N);
		}
		
		void build(int node, int b, int e)	// O(n)
		{
			if(b == e)					
				sTree[node] = array[b];
			else						
			{
				int mid = b + e >> 1;
				build(node<<1,b,mid);
				build(node<<1|1,mid+1,e);
				sTree[node] = Math.min(sTree[node<<1],sTree[node<<1|1]);
			}
		}
		
		
		void update_point(int index, int val)			// O(log n)
		{
			index += N - 1;				
			sTree[index] = val;			
			while(index>1)				
			{
				index >>= 1;
				sTree[index] = Math.min(sTree[index<<1],sTree[index<<1|1]);		
			}
		}
		
		
		void update_range(int i, int j, int val)		// O(log n) 
		{
			update_range(1,1,N,i,j,val);
		}
		
		void update_range(int node, int b, int e, int i, int j, int val)
		{
			if(i > e || j < b)		
				return;
			if(b >= i && e <= j)		
			{
				sTree[node] += (e-b+1)*val;			
				lazy[node] += val;				
			}							
			else		
			{
				int mid = b + e >> 1;
				propagate(node, b, mid, e);
				update_range(node<<1,b,mid,i,j,val);
				update_range(node<<1|1,mid+1,e,i,j,val);
				sTree[node] = sTree[node<<1] + sTree[node<<1|1];		
			}
			
		}
		void propagate(int node, int b, int mid, int e)		
		{
			lazy[node<<1] += lazy[node];
			lazy[node<<1|1] += lazy[node];
			sTree[node<<1] += (mid-b+1)*lazy[node];		
			sTree[node<<1|1] += (e-mid)*lazy[node];		
			lazy[node] = 0;
		}
		
		int query(int i, int j)
		{
			return query(1,1,N,i,j);
		}
		
		int query(int node, int b, int e, int i, int j)	// O(log n)
		{
			if(i>e || j <b)
				return 0;		
			if(b>= i && e <= j)
				return sTree[node];
			int mid = b + e >> 1;
			propagate(node, b, mid, e);
			int q1 = query(node<<1,b,mid,i,j);
			int q2 = query(node<<1|1,mid+1,e,i,j);
			return Math.min(q1,q2);	
					
		}
		
		
	}



	static class UnionFind {
		int[] p, rank, setSize;
		int numSets;
		int max[];

		public UnionFind(int N) {
			p = new int[numSets = N];
			rank = new int[N];
			setSize = new int[N];
			for (int i = 0; i < N; i++) {
				p[i] = i;
				setSize[i] = 1;
			}
		}

		public int findSet(int i) {
			return p[i] == i ? i : (p[i] = findSet(p[i]));
		}

		public boolean isSameSet(int i, int j) {
			return findSet(i) == findSet(j);
		}
		public int chunion(int i,int j, int x2) {
			if (isSameSet(i, j))
				return 0;
			numSets--;
			int x = findSet(i), y = findSet(j);
			int z=findSet(x2);
			p[x]=z;;
			p[y]=z;
		return x;
		}

		public void unionSet(int i, int j) {
			if (isSameSet(i, j))
				return;
			numSets--;
			int x = findSet(i), y = findSet(j);
			if (rank[x] > rank[y]) {
				p[y] = x;
				setSize[x] += setSize[y];


			} else {
				p[x] = y;
				setSize[y] += setSize[x];
				if (rank[x] == rank[y])
					rank[y]++;


			}
		}

		

		public int numDisjointSets() {
			return numSets;
		}

		public int sizeOfSet(int i) {
			return setSize[findSet(i)];
		}
	}

	/**
	 * private static void trace(int i, int time) { if(i==n) return;
	 * 
	 * 
	 * long ans=dp(i,time);
	 * if(time+a[i].t<a[i].burn&&(ans==dp(i+1,time+a[i].t)+a[i].cost)) {
	 * 
	 * trace(i+1, time+a[i].t);
	 * 
	 * l1.add(a[i].idx); return; } trace(i+1,time);
	 * 
	 * }
	 **/

	static class incpair implements Comparable<incpair> {
		int a;
		long b;
		int idx;

		incpair(int a, long dirg, int i) {
			this.a = a;
			b = dirg;
			idx = i;
		}

		public int compareTo(incpair e) {
			return (int) (b - e.b);
		}
	}

	static class decpair implements Comparable<decpair> {
		int a;
		long b;
		int idx;

		decpair(int a, long dirg, int i) {
			this.a = a;
			b = dirg;
			idx = i;
		}

		public int compareTo(decpair e) {
			return (int) (e.b - b);
		}
	}

	static long allpowers[];

	static class Quad implements Comparable<Quad> {
		int u;
		int v;
		char state;
		int turns;

		public Quad(int i, int j, char c, int k) {
			u = i;
			v = j;
			state = c;
			turns = k;
		}

		public int compareTo(Quad e) {
			return (int) (turns - e.turns);
		}

	}

	static long dirg[][];
	static Edge[] driver;

	static int n;

	static long manhatandistance(long x, long x2, long y, long y2) {
		return Math.abs(x - x2) + Math.abs(y - y2);
	}

	static long fib[];

	static long fib(int n) {
		if (n == 1 || n == 0) {
			return 1;
		}
		if (fib[n] != -1) {
			return fib[n];
		} else
			return fib[n] = ((fib(n - 2) % mod + fib(n - 1) % mod) % mod);
	}

	static class Point  implements Comparable<Point>{
		long x, y;

		Point(long counth, long counts) {
			x = counth;
			y = counts;
		}

		@Override
		public int compareTo(Point p )
		{
			return Long.compare(p.y*1l*x, p.x*1l*y);
		}
		


	}

	static long[][] comb;

	static class Triple implements Comparable<Triple> {

		int l;
		int r;
		long cost;
		int idx;

		public Triple(int a, int b, long l1, int l2) {
			l = a;
			r = b;
			cost = l1;
			idx = l2;
		}

		public int compareTo(Triple x) {
			if (l != x.l || idx == x.idx)
				return l - x.l;
			return -idx;
		}

	}

	static TreeSet<Long> primeFactors(long N) // O(sqrt(N) / ln sqrt(N))
	{
		TreeSet<Long> factors = new TreeSet<Long>(); // take abs(N) in case of -ve integers
		int idx = 0, p = primes.get(idx);

		while (p * p <= N) {
			while (N % p == 0) {
				factors.add((long) p);
				N /= p;
			}
			if (primes.size() > idx + 1)
				p = primes.get(++idx);
			else
				break;
		}

		if (N != 1) // last prime factor may be > sqrt(N)
			factors.add(N); // for integers whose largest prime factor has a power of 1
		return factors;
	}

	static boolean visited[];

	/**
	 * static int bfs(int s) { Queue<Integer> q = new LinkedList<Integer>();
	 * q.add(s); int count=0; int maxcost=0; int dist[]=new int[n]; dist[s]=0;
	 * while(!q.isEmpty()) {
	 * 
	 * int u = q.remove(); if(dist[u]==k) { break; } for(Pair v: adj[u]) {
	 * maxcost=Math.max(maxcost, v.cost);
	 * 
	 * 
	 * 
	 * if(!visited[v.v]) {
	 * 
	 * visited[v.v]=true; q.add(v.v); dist[v.v]=dist[u]+1; maxcost=Math.max(maxcost,
	 * v.cost); } }
	 * 
	 * } return maxcost; }
	 **/
	public static boolean FindAllElements(int n, int k) {
		int sum = k;
		int[] A = new int[k];
		Arrays.fill(A, 0, k, 1);

		for (int i = k - 1; i >= 0; --i) {

			while (sum + A[i] <= n) {

				sum += A[i];
				A[i] *= 2;
			}
		}
		if (sum == n) {
			return true;
		} else
			return false;
	}

	static boolean[] vis2;

	static boolean f2 = false;

	static long[][] matMul(long[][] a2, long[][] b, int p, int q, int r) // C(p x r) = A(p x q) x (q x r) -- O(p x q x
																			// r)
	{
		long[][] C = new long[p][r];
		for (int i = 0; i < p; ++i) {
			for (int j = 0; j < r; ++j) {
				for (int k = 0; k < q; ++k) {
					C[i][j] = (C[i][j] + (a2[i][k] % mod * b[k][j] % mod)) % mod;
					C[i][j] %= mod;
				}

			}
		}
		return C;
	}
	public static int[] schuffle(int[] a2) {
		for (int i = 0; i < a2.length; i++) {
			int x =  (int) (Math.random() * a2.length);
			int temp = a2[x];
			a2[x] = a2[i];
			a2[i] = temp;
		}
		return a2;
	}

	static int memo1[];

	static boolean vis[];
	static TreeSet<Integer> set = new TreeSet<Integer>();

	static long modPow(long ways, long count, long mod) // O(log e)
	{
		ways %= mod;
		long res = 1;
		while (count > 0) {
			if ((count & 1) == 1)
				res = (res * ways) % mod;
			ways = (ways * ways) % mod;
			count >>= 1;
		}
		return res % mod;
	}

	static int gcd(int ans, int b) {
		if (b == 0) {
			return ans;
		}
		return gcd(b, ans % b);
	}

	static int[] isComposite;
	static int[] valid;

	static ArrayList<Integer> primes;
	static ArrayList<Integer> l1;

	
	static TreeSet<Integer> primus = new TreeSet<Integer>();
	
	static void sieveLinear(int N)
	{
		int[] lp = new int[N + 1];								//lp[i] = least prime divisor of i
		for(int i = 2; i <= N; ++i)
		{
			if(lp[i] == 0)
			{
				primus.add(i);
				lp[i] = i;
			}
			int curLP = lp[i];
			for(int p: primus)
				if(p > curLP || p * i > N)
					break;
				else
					lp[p * i] = i;
		}
	}
	

	public static long[] schuffle(long[] a2) {
		for (int i = 0; i < a2.length; i++) {
			int x =  (int) (Math.random() * a2.length);
			long temp = a2[x];
			a2[x] = a2[i];
			a2[i] = temp;
		}
		return a2;
	}

	static int V;
	static long INF = (long) 1E16;

	static class Edge2 {
		int node;
		long cost;
		long next;

		Edge2(int a, int c, Long long1) {
			node = a;
			cost = long1;
			next = c;
		}

	}

	static class Scanner {
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream system) {
			br = new BufferedReader(new InputStreamReader(system));
		}

		public Scanner(String file) throws Exception {
			br = new BufferedReader(new FileReader(file));
		}

		public String next() throws IOException {
			while (st == null || !st.hasMoreTokens())
				st = new StringTokenizer(br.readLine());
			return st.nextToken();
		}

		public String nextLine() throws IOException {
			return br.readLine();
		}

		public int nextInt() throws IOException {
			return Integer.parseInt(next());
		}

		public double nextDouble() throws IOException {
			return Double.parseDouble(next());
		}

		public char nextChar() throws IOException {
			return next().charAt(0);
		}

		public Long nextLong() throws IOException {
			return Long.parseLong(next());
		}

		public boolean ready() throws IOException {
			return br.ready();
		}

		public void waitForInput() throws InterruptedException {
			Thread.sleep(3000);
		}

		public int[] nxtArr(int n) throws IOException {
			int[] ans = new int[n];
			for (int i = 0; i < n; i++)
				ans[i] = nextInt();
			return ans;
		}
		
	}
	public static int[] sortarray(int a[]) {
		schuffle(a);
		Arrays.sort(a);
		return a;
		}
		public static long[] sortarray(long a[]) {
			schuffle(a);
			Arrays.sort(a);
			return a;
		}
}