# Mutual Followers

You are given a two-dimensional list of integers `relations`

. Each element `relations[i]`

contains `[a, b]`

meaning that person `a`

is following person `b`

on Twitter.

Return the list of people who follow someone that follows them back, sorted in ascending order.

**Constraints**

`0 ≤ n ≤ 100,000`

where`n`

is the length of`relations`

https://binarysearch.com/problems/Mutual-Followers

## Examples

### Example 1

**Input**

- relations =

```
[[0,1],
[2,3],
[3,4],
[1,0]]
```

**Output**

- answer =
`[0, 1]`

**Explanation**

`0`

follows `1`

and `1`

follows `0`

.

### Example 2

**Input**

- relations =

```
[[0,1],
[1,2],
[2,3],
[3,0]]
```

**Output**

- answer =
`[]`

**Explanation**

There aren’t any mutual followers.

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