题解与来源题共享。已有题解默认通过；新增或编辑后需由来源域管理员审核。待审核、驳回题解仅作者和审核管理员可见。来源域：system

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  Rose LV 4 @ 2026-3-10 14:54:20

  n = int(input()) a = list(map(int, input().split()))

  pre_sum = [0] * (n + 1) for i in range(1, n + 1): pre_sum[i] = pre_sum[i - 1] + a[i - 1]

  total = pre_sum[n] if total % 3 != 0: print(0) else: target = total // 3 res = 0 count = 0

  for j in range(1, n): 
      if pre_sum[j] == 2 * target:
          res += count
      if pre_sum[j] == target:
          count += 1
  
  print(res)