Score : $300$ points

问题描述

给定一个长度为 $N$ 的整数序列 $a = (a_1, \dots, a_N)$，其中的整数取值范围在 $1$ 到 $N$ 之间。

找出满足以下所有条件的整数对 $(i, j)$ 的数量：

• $1 \leq i < j \leq N$
• $\min(a_i, a_j) = i$
• $\max(a_i, a_j) = j$

以上为通义千问 qwen-max 翻译，仅供参考。

Problem Statement

You are given a sequence $a = (a_1, \dots, a_N)$ of length $N$ consisting of integers between $1$ and $N$.

Find the number of pairs of integers $i, j$ that satisfy all of the following conditions:

• $1 \leq i \lt j \leq N$
• $\min(a_i, a_j) = i$
• $\max(a_i, a_j) = j$

Constraints

• $2 \leq N \leq 5 \times 10^5$
• $1 \leq a_i \leq N \, (1 \leq i \leq N)$
• All values in input are integers.

Input

Input is given from Standard Input in the following format:

$N$

$a_1$ $\ldots$ $a_N$

Output

Print the answer.

Sample Input 1

4
1 3 2 4

Sample Output 1

2

$(i, j) = (1, 4), (2, 3)$ satisfy the conditions.

Sample Input 2

10
5 8 2 2 1 6 7 2 9 10

Sample Output 2

8

update @ 2024/3/10 11:05:56