### PROBLEM LINK:

**Author:** Vipul Sharma

**Tester:** Prateek Gupta

**Editorialist:** Oleksandr Kulkov

### DIFFICULTY:

MEDIUM

### PREREQUISITES:

None

### PROBLEM:

You are given three arrays A, B and C. You have to sum up (A_i+B_j)(B_j+C_k) for all i, j, k such that A_i \leq B_j \geq C_k.

### QUICK EXPLANATION:

Over all B_j's, sum up \left(\sum\limits_{A_i \leq B_j}(A_i+B_j)\right) imes\left(\sum\limits_{C_k \geq B_j}(B_j+C_k)\right).

### EXPLANATION:

Let’s bruteforce B_j. If we fix some j, then we can choose A_i \leq B_j and C_k \geq B_j independently. Let’s sort A in ascending order and C in descending order. Then suitable A_i as well as suitable C_k will form prefix of A and C correspondingly which size can be found by binary search. Let the sizes of these prefixes will be k_1 and k_2. Then from triples with this B_j we will have contribution of \sum\limits_{i=1}^{k_1}\sum\limits_{k=1}^{k_2}(A_i+B_j)(B_j+C_k)=\sum\limits_{i=1}^{k_1}(A_i+B_j)\left(k_2 B_j+\sum\limits_{k=1}^{k_2}C_k\right)=\left(k_1 B_j+\sum\limits_{i=1}^{k_1}A_i\right) imes \left(k_2 B_j+\sum\limits_{k=1}^{k_2}C_k\right) which can be quickly calculated using prefix sums. Thus we have O(n \log n) solution.

We can optimize this solution to \mathcal{O}(n) by making the following observation. Let us sort the B in increasing order. The values of k_1 and k_2 can only increase when we go from left to right in the array B. This technique’s standard name is “two pointers” approach.

### AUTHOR’S AND TESTER’S SOLUTIONS:

Author’s solution will be updated soon.

Tester’s solution will be updated soon.