题解 | 数对计数

import sys
from collections import defaultdict

def main():
    data = sys.stdin.read().split()
    n = int(data[0])
    c = int(data[1])
    arr = list(map(int, data[2:2+n]))
    
    count_dict = defaultdict(int)
    for num in arr:
        count_dict[num] += 1
        
    res = 0
    for num in count_dict:
        target = num + c
        if target in count_dict:
            res += count_dict[num] * count_dict[target]
            
    print(res)

if __name__ == "__main__":
    main()