题解 | 特殊城市

import sys
from collections import defaultdict
# 读取所有输入，处理大量数据更高效
data = sys.stdin.read().split()
ptr = 0
N = int(data[ptr])
ptr += 1

# 统计 (prefix, state) 组合的出现次数
combo_count = defaultdict(int)

for _ in range(N):
    city = data[ptr]
    state = data[ptr + 1]
    # 提取城市名称前两位作为前缀
    prefix = city[:2]
    combo_count[(prefix, state)] += 1
    ptr += 2

result = 0
# 遍历所有唯一的 (prefix, state) 组合
for (prefix_a, state_a), count_a in combo_count.items():
    # 特殊对的反向组合是 (state_a, prefix_a)
    reverse_combo = (state_a, prefix_a)
    # 检查反向组合是否存在，且两个组合的州不同（prefix_a != state_a 保证不同州）
    if reverse_combo in combo_count and prefix_a != state_a:
        count_b = combo_count[reverse_combo]
        result += count_a * count_b

# 每对城市被计算了两次（A-B 和 B-A），所以除以2
print(result // 2)