Off-by-one on range boundaries
Wrong move: Loop endpoints miss first/last candidate.
Usually fails on: Fails on minimal arrays and exact-boundary answers.
Fix: Re-derive loops from inclusive/exclusive ranges before coding.
Break down a hard problem into reliable checkpoints, edge-case handling, and complexity trade-offs.
You are given an integer array nums. We call a subset of nums good if its product can be represented as a product of one or more distinct prime numbers.
nums = [1, 2, 3, 4]:
[2, 3], [1, 2, 3], and [1, 3] are good subsets with products 6 = 2*3, 6 = 2*3, and 3 = 3 respectively.[1, 4] and [4] are not good subsets with products 4 = 2*2 and 4 = 2*2 respectively.Return the number of different good subsets in nums modulo 109 + 7.
A subset of nums is any array that can be obtained by deleting some (possibly none or all) elements from nums. Two subsets are different if and only if the chosen indices to delete are different.
Example 1:
Input: nums = [1,2,3,4] Output: 6 Explanation: The good subsets are: - [1,2]: product is 2, which is the product of distinct prime 2. - [1,2,3]: product is 6, which is the product of distinct primes 2 and 3. - [1,3]: product is 3, which is the product of distinct prime 3. - [2]: product is 2, which is the product of distinct prime 2. - [2,3]: product is 6, which is the product of distinct primes 2 and 3. - [3]: product is 3, which is the product of distinct prime 3.
Example 2:
Input: nums = [4,2,3,15] Output: 5 Explanation: The good subsets are: - [2]: product is 2, which is the product of distinct prime 2. - [2,3]: product is 6, which is the product of distinct primes 2 and 3. - [2,15]: product is 30, which is the product of distinct primes 2, 3, and 5. - [3]: product is 3, which is the product of distinct prime 3. - [15]: product is 15, which is the product of distinct primes 3 and 5.
Constraints:
1 <= nums.length <= 1051 <= nums[i] <= 30Problem summary: You are given an integer array nums. We call a subset of nums good if its product can be represented as a product of one or more distinct prime numbers. For example, if nums = [1, 2, 3, 4]: [2, 3], [1, 2, 3], and [1, 3] are good subsets with products 6 = 2*3, 6 = 2*3, and 3 = 3 respectively. [1, 4] and [4] are not good subsets with products 4 = 2*2 and 4 = 2*2 respectively. Return the number of different good subsets in nums modulo 109 + 7. A subset of nums is any array that can be obtained by deleting some (possibly none or all) elements from nums. Two subsets are different if and only if the chosen indices to delete are different.
Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.
Pattern signal: Array · Hash Map · Math · Dynamic Programming · Bit Manipulation
[1,2,3,4]
[4,2,3,15]
smallest-sufficient-team)fair-distribution-of-cookies)number-of-ways-to-wear-different-hats-to-each-other)Source-backed implementations are provided below for direct study and interview prep.
// Accepted solution for LeetCode #1994: The Number of Good Subsets
class Solution {
public int numberOfGoodSubsets(int[] nums) {
int[] primes = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29};
int[] cnt = new int[31];
for (int x : nums) {
++cnt[x];
}
final int mod = (int) 1e9 + 7;
int n = primes.length;
long[] f = new long[1 << n];
f[0] = 1;
for (int i = 0; i < cnt[1]; ++i) {
f[0] = (f[0] * 2) % mod;
}
for (int x = 2; x < 31; ++x) {
if (cnt[x] == 0 || x % 4 == 0 || x % 9 == 0 || x % 25 == 0) {
continue;
}
int mask = 0;
for (int i = 0; i < n; ++i) {
if (x % primes[i] == 0) {
mask |= 1 << i;
}
}
for (int state = (1 << n) - 1; state > 0; --state) {
if ((state & mask) == mask) {
f[state] = (f[state] + cnt[x] * f[state ^ mask]) % mod;
}
}
}
long ans = 0;
for (int i = 1; i < 1 << n; ++i) {
ans = (ans + f[i]) % mod;
}
return (int) ans;
}
}
// Accepted solution for LeetCode #1994: The Number of Good Subsets
func numberOfGoodSubsets(nums []int) (ans int) {
primes := []int{2, 3, 5, 7, 11, 13, 17, 19, 23, 29}
cnt := [31]int{}
for _, x := range nums {
cnt[x]++
}
const mod int = 1e9 + 7
n := 10
f := make([]int, 1<<n)
f[0] = 1
for i := 0; i < cnt[1]; i++ {
f[0] = f[0] * 2 % mod
}
for x := 2; x < 31; x++ {
if cnt[x] == 0 || x%4 == 0 || x%9 == 0 || x%25 == 0 {
continue
}
mask := 0
for i, p := range primes {
if x%p == 0 {
mask |= 1 << i
}
}
for state := 1<<n - 1; state > 0; state-- {
if state&mask == mask {
f[state] = (f[state] + f[state^mask]*cnt[x]) % mod
}
}
}
for i := 1; i < 1<<n; i++ {
ans = (ans + f[i]) % mod
}
return
}
# Accepted solution for LeetCode #1994: The Number of Good Subsets
class Solution:
def numberOfGoodSubsets(self, nums: List[int]) -> int:
primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
cnt = Counter(nums)
mod = 10**9 + 7
n = len(primes)
f = [0] * (1 << n)
f[0] = pow(2, cnt[1])
for x in range(2, 31):
if cnt[x] == 0 or x % 4 == 0 or x % 9 == 0 or x % 25 == 0:
continue
mask = 0
for i, p in enumerate(primes):
if x % p == 0:
mask |= 1 << i
for state in range((1 << n) - 1, 0, -1):
if state & mask == mask:
f[state] = (f[state] + cnt[x] * f[state ^ mask]) % mod
return sum(f[i] for i in range(1, 1 << n)) % mod
// Accepted solution for LeetCode #1994: The Number of Good Subsets
/**
* [1994] The Number of Good Subsets
*
* You are given an integer array nums. We call a subset of nums good if its product can be represented as a product of one or more distinct prime numbers.
*
* For example, if nums = [1, 2, 3, 4]:
*
* [2, 3], [1, 2, 3], and [1, 3] are good subsets with products 6 = 2*3, 6 = 2*3, and 3 = 3 respectively.
* [1, 4] and [4] are not good subsets with products 4 = 2*2 and 4 = 2*2 respectively.
*
*
*
* Return the number of different good subsets in nums modulo 10^9 + 7.
* A subset of nums is any array that can be obtained by deleting some (possibly none or all) elements from nums. Two subsets are different if and only if the chosen indices to delete are different.
*
* Example 1:
*
* Input: nums = [1,2,3,4]
* Output: 6
* Explanation: The good subsets are:
* - [1,2]: product is 2, which is the product of distinct prime 2.
* - [1,2,3]: product is 6, which is the product of distinct primes 2 and 3.
* - [1,3]: product is 3, which is the product of distinct prime 3.
* - [2]: product is 2, which is the product of distinct prime 2.
* - [2,3]: product is 6, which is the product of distinct primes 2 and 3.
* - [3]: product is 3, which is the product of distinct prime 3.
*
* Example 2:
*
* Input: nums = [4,2,3,15]
* Output: 5
* Explanation: The good subsets are:
* - [2]: product is 2, which is the product of distinct prime 2.
* - [2,3]: product is 6, which is the product of distinct primes 2 and 3.
* - [2,15]: product is 30, which is the product of distinct primes 2, 3, and 5.
* - [3]: product is 3, which is the product of distinct prime 3.
* - [15]: product is 15, which is the product of distinct primes 3 and 5.
*
*
* Constraints:
*
* 1 <= nums.length <= 10^5
* 1 <= nums[i] <= 30
*
*/
pub struct Solution {}
// problem: https://leetcode.com/problems/the-number-of-good-subsets/
// discuss: https://leetcode.com/problems/the-number-of-good-subsets/discuss/?currentPage=1&orderBy=most_votes&query=
// submission codes start here
impl Solution {
pub fn number_of_good_subsets(nums: Vec<i32>) -> i32 {
0
}
}
// submission codes end
#[cfg(test)]
mod tests {
use super::*;
#[test]
#[ignore]
fn test_1994_example_1() {
let nums = vec![1, 2, 3, 4];
let result = 6;
assert_eq!(Solution::number_of_good_subsets(nums), result);
}
#[test]
#[ignore]
fn test_1994_example_2() {
let nums = vec![4, 2, 3, 15];
let result = 5;
assert_eq!(Solution::number_of_good_subsets(nums), result);
}
}
// Accepted solution for LeetCode #1994: The Number of Good Subsets
// Auto-generated TypeScript example from java.
function exampleSolution(): void {
}
// Reference (java):
// // Accepted solution for LeetCode #1994: The Number of Good Subsets
// class Solution {
// public int numberOfGoodSubsets(int[] nums) {
// int[] primes = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29};
// int[] cnt = new int[31];
// for (int x : nums) {
// ++cnt[x];
// }
// final int mod = (int) 1e9 + 7;
// int n = primes.length;
// long[] f = new long[1 << n];
// f[0] = 1;
// for (int i = 0; i < cnt[1]; ++i) {
// f[0] = (f[0] * 2) % mod;
// }
// for (int x = 2; x < 31; ++x) {
// if (cnt[x] == 0 || x % 4 == 0 || x % 9 == 0 || x % 25 == 0) {
// continue;
// }
// int mask = 0;
// for (int i = 0; i < n; ++i) {
// if (x % primes[i] == 0) {
// mask |= 1 << i;
// }
// }
// for (int state = (1 << n) - 1; state > 0; --state) {
// if ((state & mask) == mask) {
// f[state] = (f[state] + cnt[x] * f[state ^ mask]) % mod;
// }
// }
// }
// long ans = 0;
// for (int i = 1; i < 1 << n; ++i) {
// ans = (ans + f[i]) % mod;
// }
// return (int) ans;
// }
// }
Use this to step through a reusable interview workflow for this problem.
Pure recursion explores every possible choice at each step. With two choices per state (take or skip), the decision tree has 2ⁿ leaves. The recursion stack uses O(n) space. Many subproblems are recomputed exponentially many times.
Each cell in the DP table is computed exactly once from previously solved subproblems. The table dimensions determine both time and space. Look for the state variables — each unique combination of state values is one cell. Often a rolling array can reduce space by one dimension.
Review these before coding to avoid predictable interview regressions.
Wrong move: Loop endpoints miss first/last candidate.
Usually fails on: Fails on minimal arrays and exact-boundary answers.
Fix: Re-derive loops from inclusive/exclusive ranges before coding.
Wrong move: Zero-count keys stay in map and break distinct/count constraints.
Usually fails on: Window/map size checks are consistently off by one.
Fix: Delete keys when count reaches zero.
Wrong move: Temporary multiplications exceed integer bounds.
Usually fails on: Large inputs wrap around unexpectedly.
Fix: Use wider types, modular arithmetic, or rearranged operations.
Wrong move: An incomplete state merges distinct subproblems and caches incorrect answers.
Usually fails on: Correctness breaks on cases that differ only in hidden state.
Fix: Define state so each unique subproblem maps to one DP cell.