# Maximum Pairwise Modular Sum solution codechef

## Maximum Pairwise Modular Sum solution codechef

You are given an array AA containing NN integers, and a positive integer MM. Find the maximum value of

Ai+Aj+((AiAj)modM)Ai+Aj+((Ai−Aj)modM)

across all pairs 1i,jN1≤i,j≤N.

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Note that xmodMxmodM refers to the smallest non-negative integer obtained as the remainder upon dividing xx by MM. For example, 4mod3=14mod3=1 and (10)mod3=2(−10)mod3=2.

## Maximum Pairwise Modular Sum solution codechef

• The first line of input will contain a single integer TT, the number of test cases. The description of test cases follows.
• Each test case consists of two lines of input.
• The first line of each test case contains two space-separated integers NN and MM.
• The second line of each test case contains NN space-separated integers A1,A2,,ANA1,A2,…,AN.

### Output Format

• For each test case, output on a new line the maximum value of Ai+Aj+((AiAj)modM)Ai+Aj+((Ai−Aj)modM).

## Maximum Pairwise Modular Sum solution codechef

• 1T1001≤T≤100
• 2N21052≤N≤2⋅105
• 2M51082≤M≤5⋅108
• 0Ai51080≤Ai≤5⋅108
• The sum of NN across all test cases won’t exceed 21052⋅105.

• The sum of NN across all test cases won’t exceed 10001000
• 2M10002≤M≤1000
• Original constraints

### Sample Input 1

4
2 18
12 1
3 5
4 5 6
5 4
79 29 80 58 80
3 20
33 46 56


## Maximum Pairwise Modular Sum solution codechef

24
15
162
112


### Explanation

Test case 11: There are 44 possible pairs of indices to choose from. Their respective values are:

• i=1,j=1i=1,j=1, giving 12+12+((1212)mod18)=24+0=2412+12+((12−12)mod18)=24+0=24
• i=1,j=2i=1,j=2, giving 12+1+((121)mod18)=13+11=2412+1+((12−1)mod18)=13+11=24
• i=2,j=1i=2,j=1, giving 1+12+((112)mod18)=13+7=201+12+((1−12)mod18)=13+7=20
• i=2,j=2i=2,j=2, giving 1+1+((11)mod18)=2+0=21+1+((1−1)mod18)=2+0=2

Of these, the largest value is 2424.

Test case 22: There are 3×3=93×3=9 choices for pairs (i,j)(i,j). Of these, one way to achieve the maximum is by choosing i=2,j=3i=2,j=3, giving 5+6+((56)mod5)=11+4=155+6+((5−6)mod5)=11+4=15.

Test case 33: Picking i=1,j=3i=1,j=3 gives a value of 79+80+((7980)mod4)=159+3=16279+80+((79−80)mod4)=159+3=162, which is the largest possible.

Test case 44: Picking i=3,j=2i=3,j=2 gives a value of 56+46+((5646)mod20)=102+10=11256+46+((56−46)mod20)=102+10=112, which is the largest possible.