Problem:
Score : 600 points

Problem Statement

There are N slimes standing on a number line.
The i-th slime from the left is at position x_i.

It is guaruanteed that 1 \leq x_1 < x_2 < \ldots < x_N \leq 10^{9}.

Niwango will perform N-1 operations. The i-th operation consists of the following procedures:

Choose an integer k between 1 and N-i (inclusive) with equal probability.

Move the k-th slime from the left, to the position of the neighboring slime to the right.

Fuse the two slimes at the same position into one slime.

Find the total distance traveled by the slimes multiplied by (N-1)! (we can show that this value is an integer), modulo (10^{9}+7). If a slime is born by a fuse and that slime moves, we count it as just one slime.

Constraints

2 \leq N \leq 10^{5}

1 \leq x_1 < x_2 < \ldots < x_N \leq 10^{9}

x_i is an integer.

Subtasks

400 points will be awarded for passing the test cases satisfying N \leq 2000.

Input

Input is given from Standard Input in the following format:

N
x_1 x_2 \ldots x_N

Output

Print the answer.

Sample Input 1

3
1 2 3

Sample Output 1

5

With probability \frac{1}{2}, the leftmost slime is chosen in the first operation, in which case the total distance traveled is 2.

With probability \frac{1}{2}, the middle slime is chosen in the first operation, in which case the total distance traveled is 3.

The answer is the expected total distance traveled, 2.5, multiplied by 2!, which is 5.

Sample Input 2

12
161735902 211047202 430302156 450968417 628894325 707723857 731963982 822804784 880895728 923078537 971407775 982631932

Sample Output 2

750927044

Find the expected value multiplied by (N-1)!, modulo (10^9+7).

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)


import java.util.*;
import java.io.*;
class Main {
    static long[] factrial;
    static long[] inverse;
    static long MOD;
    public static void main(String[] args) {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        long mod = (long)(1e9+7);
        long[] a = new long[N];
        long[] b = new long[N-1];
        for(int i = 0; i<N; i++) {
            a[i] = sc.nextLong();
        }
        for(int i = 0; i<N-1; i++) {
            b[i] = (a[i+1] - a[i])%mod;
        }
        int max = (int)(3e5);
        factrial= new long[max]; //階乗を保持
        inverse= new long[max];  //逆元を保持
        MOD = mod;
        factrial[0] = 1;
        inverse[0] = 1;
            
        for (int i = 1; i < max; i++) {
            factrial[i] = factrial[i - 1] * i;  //階乗を求める
            factrial[i]%=MOD;
            inverse[i] = pow(i, (int)MOD - 2); 
        }
    
        long kake = 0;
        long ans = 0;
        for(int i = 0; i<N-1; i++) {
            kake += inverse[i+1]%mod;
            pe((i+1)+" "+inverse[i+1]);
            kake %= mod;
            //pe(kake);
            ans += b[i]*kake%mod;
            ans %= mod;
            //pe("ans "+ans);
        }
        ans *= factrial[N-1]%mod;
	p(ans%mod);
    }
    
    public static long combine(int n, int k) {
        return factrial[n] * inverse[k] % MOD * inverse[n - k] % MOD;
    }
    
    public static long pow(long x, int n) { //x^n 計算量O(logn)
		long ans = 1;
		while (n > 0) {
			if ((n & 1) == 1) {
				ans = ans * x % MOD;
			}
			x = x * x % MOD; //一周する度にx, x^2, x^4, x^8となる
			n >>= 1; //桁をずらす n = n >> 1
		}
		return ans;
	}
    public static <T> void p(T element) {
	System.out.println(element);
    }
    public static <T> void pe(T element) {
        System.err.println(element);
        }
    public static int min(int a, int b, int c) {
	int min = Math.min(a,b);
	min = Math.min(min,c);
	return min;
    }
    public static int max(int a, int b,int c) {
	int max= Math.max(a,b);
	max = Math.max(max,c);
        return max;
    }
}

class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
 
    private int readByte() {
        if (hasNextByte())
            return buffer[ptr++];
        else
            return -1;
    }
 
    private boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }
 
    public boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr]))
            ptr++;
        return hasNextByte();
    }
 
    public String next() {
        if (!hasNext())
            throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
 
    public long nextLong() {
        if (!hasNext())
            throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if (b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
 
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }
 
    public double nextDouble() {
        return Double.parseDouble(next());
    }
}