# 2001. Number of Pairs of Interchangeable Rectangles

<https://leetcode.com/problems/number-of-pairs-of-interchangeable-rectangles>

## Description

You are given `n` rectangles represented by a **0-indexed** 2D integer array `rectangles`, where `rectangles[i] = [widthi, heighti]` denotes the width and height of the `ith` rectangle.

Two rectangles `i` and `j` (`i < j`) are considered **interchangeable** if they have the **same** width-to-height ratio. More formally, two rectangles are **interchangeable** if `widthi/heighti == widthj/heightj` (using decimal division, not integer division).

Return *the **number** of pairs of **interchangeable** rectangles in* `rectangles`.

**Example 1:**

```
**Input:** rectangles = [[4,8],[3,6],[10,20],[15,30]]
**Output:** 6
**Explanation:** The following are the interchangeable pairs of rectangles by index (0-indexed):
- Rectangle 0 with rectangle 1: 4/8 == 3/6.
- Rectangle 0 with rectangle 2: 4/8 == 10/20.
- Rectangle 0 with rectangle 3: 4/8 == 15/30.
- Rectangle 1 with rectangle 2: 3/6 == 10/20.
- Rectangle 1 with rectangle 3: 3/6 == 15/30.
- Rectangle 2 with rectangle 3: 10/20 == 15/30.
```

**Example 2:**

```
**Input:** rectangles = [[4,5],[7,8]]
**Output:** 0
**Explanation:** There are no interchangeable pairs of rectangles.
```

**Constraints:**

* `n == rectangles.length`
* `1 <= n <= 105`
* `rectangles[i].length == 2`
* `1 <= widthi, heighti <= 105`

## ac

```java
```


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