题解 | 数对计数

n, c = map(int,input().split())
nums = list(map(int,input().split()))

counts = {}
for num in nums:#统计输入数据中，各数据的个数，存储到字典中
    counts[num] = counts.get(num,0)+1

result = 0
for key, value in counts.items():#遍历字典，查看字典中，是否有能和当前数据匹配的数据
    target = key-c
    if target in counts:
        result += value*counts[target]

print(result)