{"problem":{"name":"COUNTARI","description":{"content":"给定一个长度为 $n$ 的数组 $a$，求有多少对 $i,j,k(1\\leq i<j<k\\leq n)$，满足 $a_k-a_j=a_j-a_i$。","description_type":"Markdown"},"platform":"Luogu","limit":{"time_limit":3000,"memory_limit":524288},"difficulty":{"LuoguStyle":"P6"},"is_remote":true,"is_sync":true,"sync_url":null,"sign":"LGP10666"},"statements":[{"statement_type":"Markdown","content":"给定一个长度为 $n$ 的数组 $a$，求有多少对 $i,j,k(1\\leq i<j<k\\leq n)$，满足 $a_k-a_j=a_j-a_i$。\n\n## Input\n\n第一行一个整数 $n$，第二行 $n$ 个正整数 $a_i$。\n\n## Output\n\n一行一个整数表示所求的结果。\n\n[samples]\n\n## Note\n\n数据保证，$1\\leq n\\leq 10^5$，$1\\leq a_i\\leq 3\\times 10^4$。","is_translate":false,"language":"English"}],"meta":{"iden":"LGP10666","tags":["O2优化","分块","快速傅里叶变换 FFT"],"sample_group":[["10\n3 5 3 6 3 4 10 4 5 2","9"]],"created_at":"2026-03-03 11:09:25"}}