[M hash] lc1726. Same product tuple (hash+count+weekly contest 224_2)

1. Source of the subject

Link: 1726. Tuples of the same product

2. Topic analysis

Counting problem. I didn't see the question clearly, and kept thinking about how to remove the repetition. As a result, the question required no repetitive elements... The weekly competition was quite unsuccessful.

Ideas:

• The hash table records the number of occurrences of all products.
• Product appears ktimes, would constitute $8\times C_k^2$The same product tuples. Where $C_k^2=\frac{k(k-1)}{2}$。
• For each product, find the corresponding number of tuples of the same product.

---

• Time complexity : $O(n^2)$。
• Space complexity : $O\left(n\right)$

Code:

class Solution {
    
    
public:
    int tupleSameProduct(vector<int>& nums) {
    
    
        int n = nums.size();
        unordered_map<int, int> hash;
        for (int i = 0; i < n; i ++ ) 
            for (int j = i + 1; j < n; j ++ ) 
                hash[nums[i] * nums[j]] ++ ;
        int res = 0;
        for (auto& [p, k] : hash) 
            res += k * (k - 1) / 2 * 8;
        return res;
    }
};