3 Sum With Multiplicity

Given an integer array arr, and an integer target, return the number of tuples i, j, k such that i < j < k and arr[i] + arr[j] + arr[k] == target.

As the answer can be very large, return it modulo 109 + 7.

Example 1:

Input: arr = [1,1,2,2,3,3,4,4,5,5], target = 8 .
Output: 20 
Explanation:  Enumerating by the values (arr[i], arr[j], arr[k]): (1, 2, 5) occurs 8 times; (1, 3, 4) occurs 8 times; (2, 2, 4) occurs 2 times; (2, 3, 3) occurs 2 times. 

Example 2:

Input: arr = [1,1,2,2,2,2], target = 5 
Output: 12 
Explanation:  arr[i] = 1, arr[j] = arr[k] = 2 occurs 12 times: We choose one 1 from [1,1] in 2 ways, and two 2s from [2,2,2,2] in 6 ways. 
 

Constraints:

• 3 <= arr.length <= 3000

• 0 <= arr[i] <= 100

• 0 <= target <= 300

Solution:

class Solution {
   public int threeSumMulti(int[] arr, int target) {
        Map<Integer, Long> frequency = new HashMap<>();
        for(int i: arr){
            long freq = frequency.getOrDefault(i, 0l);
            frequency.put(i, freq+1l);
        }

        long ans =0;
        for(Integer x: frequency.keySet()){
            for(Integer y : frequency.keySet()){
                int z = target -x -y;
                if(frequency.containsKey(z)){
                    long xfreq = frequency.get(x);
                    long yfreq = frequency.get(y);
                    long zfreq = frequency.get(z);

                    if(x == y && x ==z){
                        ans += ((xfreq) * (xfreq-1)*(xfreq-2))/6;
                    } else if(x ==y && x!=z){
                        ans += ((xfreq)*(xfreq-1))/2 *zfreq;
                    } else if(x< y && y<z){
                        ans += ((xfreq*yfreq *zfreq));
                    }

                }
                ans = ans % 1000000007;
            }
        }
        return (int) ans;
    }
}