题解 | 数对计数

import sys

N, C = map(int, sys.stdin.readline().split())
d, r, li = {}, 0, map(int, sys.stdin.readline().split())
for i in li:
    d[i] = d.get(i, 0) + 1      # 字典.get(键, 初始默认值)
for K, V in d.items():
    X = K - C                   # X为减数，K为被减数
    if X in d:
        r += V * d[X]           # 被减数的数量✖减数的数量
print(r)