[python] 백준 - 1253 (좋다)

import sys
input = sys.stdin.readline

n = int(input())
arr = list(map(int, input().split()))
arr.sort()

answer = 0
for i in range(n):
    num = arr[i]
    left, right = 0, n - 1

    while left < right:
        if left == i:
            left += 1
            continue
        if right == i:
            right -= 1
            continue

        if arr[left] + arr[right] == num:
            answer += 1
            break

        if arr[left] + arr[right] > num:
            right -= 1
        else:
            left += 1

print(answer)