Pairs With a Given Sum

Difficulty: 1300 3000 ms 128 MB

implementation

Count the pairs of positions i < j with a_i + a_j = K.

Input: the first line has N and K; the second line has N integers.

Output: the number of pairs summing to K.

Constraints:

• 1 ≤ N ≤ 10⁵
• −10⁹ ≤ a_i, K ≤ 10⁹

Sample Tests

5 6
1 5 7 -1 5

3

4 0
0 0 0 0

6

Read the Editorial