Alien Languages
Problem Statement :
Sophia has discovered several alien languages. Suprisingly, all of these languages have an alphabet, and each of them may contain thousands of characters! Also, all the words in a language have the same number of characters in it. However, the aliens like their words to be aesthetically pleasing, which for them means that for the ith letter of an n-letter alphabet (letters are indexed 1 . . . n ): if 2i > n, then the ith letter may be the last letter of a word, or it may be immediately followed by any letter, including itself. if 2i <= n, then the ith letter can not be the last letter of a word and also can only be immediately followed by jth letter if and only if j >=2i. Sophia wants to know how many different words exist in this language. Since the result may be large, she wants to know this number, modulo 100000007(10^8 + 7). Input Format The first line contains t, the number of test cases. The first line is followed by t lines, each line denoting a test case. Each test case will have two space-separated integers n, m which denote the number of letters in the language and the length of words in this language respectively. Constraints 1 <= t <= 5 1 <= n <= 10^5 1 <= m <= 5.10^5 Output Format For each test case, output the number of possible words modulo 100000007(10^8 + 7).
Solution :
Solution in C :
In C++ :
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
long long MOD = 100000007;
long long n, m, cnt[120000], d[120000], met[120000];
long long dd[1200000];
int main() {
int t;
cin >> t;
for(int T = 0; T < t; ++T) {
cin >> n >> m;
for(int i = 1; i <= n; ++i) {
cnt[i] = 1;
}
for(int q = 1; q < 30; ++q) {
d[q] = 0;
for(int i = 1; i <= n; ++i) {
met[i] = 0;
if (2 * i > n) {
d[q] += cnt[i];
d[q] %= MOD;
cnt[i] = 0;
}
}
for(int i = 1; i <= n; ++i) {
met[2 * i] += cnt[i];
}
long long now = 0;
for(int i = 1; i <= n; ++i) {
now += met[i];
now %= MOD;
cnt[i] = now;
}
}
for(int i = 0; i < 1200000; ++i) {
dd[i] = 0;
}
dd[0] = 1;
for(int i = 0; i < m; ++i) {
for(int j = 0; j < 30; ++j) {
dd[i + j] += (dd[i] * d[j]) % MOD;
dd[i + j] %= MOD;
}
}
cout << dd[m] % MOD << endl;
}
return 0;
}
In Java :
import java.util.*;
class Solution {
static final int MOD = 100000007;
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int cases = in.nextInt();
while(cases-->0) {
int size = in.nextInt();
int length = in.nextInt();
int[][] array = buildArray(size);
int[] eachSize = new int[array[0].length+1];
for(int i=1;i<eachSize.length;++i) eachSize[i] = array[1][i-1];
// System.err.println(Arrays.toString(eachSize));
int[] dp = new int[length+1];
dp[0] = 1;
for(int i=1;i<=length;++i) {
for(int j=1;j<eachSize.length;++j) {
if (j > i) break;
long temp = (long)eachSize[j];
temp *= dp[i-j];
temp %= MOD;
dp[i] += temp;
dp[i] %= MOD;
}
}
System.out.println(dp[length]);
}
}
public static int[][] buildArray(int size) {
// size is size of alphabet
// our array needs to be log_2(size) by size
int depth = log2(size);
int[][] array = new int[size+1][depth]; // letters are 1-indexed
for(int i=size;i>0;--i) {
// only these letters can end a word
if (i == size) array[i][0] = 1;
else if (i > size/2) array[i][0] = 1 + array[i+1][0];
else array[i][0] = array[i+1][0];
}
// build the rest of the array
for(int i=size-1;i>0;--i) {
for(int j=1;j<depth;++j) {
if (2*i > size) {
array[i][j] = array[i+1][j];
} else {
array[i][j] = array[i+1][j] + array[2*i][j-1];
// if (array[i][j] < 0) System.err.println("woop");
array[i][j] %= MOD;
}
}
}
return array;
}
// this isn't actually log2(x), since log2(1) returns 1 and not 0
// but it's what i need!
public static int log2(int x) {
int ans = 0;
while(x > 0) {
x >>= 1;
++ans;
}
return ans;
}
}
In C :
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
int dp[2][100001];
long long material[17];
int number[500001];
int reduce(long long value) {
while(value < 0)
value += 100000007;
return value % 100000007;
}
int main() {
int t, n, m, middle, i, j, sum, index = 0, first, second, max_length;
long long result;
scanf("%d", &t);
for (; t > 0; --t) {
scanf("%d %d", &n, &m);
memset(dp, 0, sizeof(dp));
memset(material, 0, sizeof(material));
middle = n / 2;
for (i = middle + 1; i <= n; ++i) {
dp[index][i] = 1;
}
for (i = 2; i <= m + 1; ++i) {
index = 1 - index;
if (i == 2) {
sum = n - middle;
} else {
sum = 0;
for (j = 1; j <= middle; ++j) {
if (dp[1 - index][j] == 0) {
break;
}
sum += dp[1 - index][j];
sum = reduce(sum);
}
}
if (sum == 0) {
break;
}
material[i - 1] = sum;
for (j = 1; j <= middle; ++j) {
first = (j << 1) - 1;
second = first - 1;
sum -= dp[1 - index][first] + dp[1 - index][second];
sum = reduce(sum);
dp[index][j] = sum;
}
for (j = middle + 1; j <= n; ++j) {
dp[index][j] = 0;
}
}
max_length = i - 2;
number[0] = 1;
for (i = 1; i <= m; ++i) {
result = 0;
for (j = 1; j <= max_length && j <= i; ++j) {
result += material[j] * number[i - j];
result = reduce(result);
}
number[i] = result;
}
printf("%d\n", number[m]);
}
return 0;
}
In Python3 :
mod = 10**8 + 7
for cas in range(int(input())):
n, m = map(int, input().strip().split())
v = [2*i > n for i in range(n+1)]
for i in range(n-1,-1,-1):
v[i] += v[i + 1]
c = []
while v[1]:
c.append(v[1])
for i in range(1,n//2+1):
v[i] = v[2*i]
for i in range(n//2+1,n+1):
v[i] = 0
for i in range(n-1,-1,-1):
v[i] = (v[i] + v[i + 1]) % mod
f = [1] + [0]*(len(c)-1)
for k in range(1,m+1):
f = [sum(F * C for F, C in zip(f, c)) % mod] + f[:-1]
print(f[0])
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