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# 1370. Increasing Decreasing String

<https://leetcode.com/problems/increasing-decreasing-string>

## Description

Given a string `s`. You should re-order the string using the following algorithm:

1. Pick the **smallest** character from `s` and **append** it to the result.
2. Pick the **smallest** character from `s` which is greater than the last appended character to the result and **append** it.
3. Repeat step 2 until you cannot pick more characters.
4. Pick the **largest** character from `s` and **append** it to the result.
5. Pick the **largest** character from `s` which is smaller than the last appended character to the result and **append** it.
6. Repeat step 5 until you cannot pick more characters.
7. Repeat the steps from 1 to 6 until you pick all characters from `s`.

In each step, If the smallest or the largest character appears more than once you can choose any occurrence and append it to the result.

Return *the result string* after sorting `s` with this algorithm.

**Example 1:**

```
**Input:** s = "aaaabbbbcccc"
**Output:** "abccbaabccba"
**Explanation:** After steps 1, 2 and 3 of the first iteration, result = "abc"
After steps 4, 5 and 6 of the first iteration, result = "abccba"
First iteration is done. Now s = "aabbcc" and we go back to step 1
After steps 1, 2 and 3 of the second iteration, result = "abccbaabc"
After steps 4, 5 and 6 of the second iteration, result = "abccbaabccba"
```

**Example 2:**

```
**Input:** s = "rat"
**Output:** "art"
**Explanation:** The word "rat" becomes "art" after re-ordering it with the mentioned algorithm.
```

**Example 3:**

```
**Input:** s = "leetcode"
**Output:** "cdelotee"
```

**Example 4:**

```
**Input:** s = "ggggggg"
**Output:** "ggggggg"
```

**Example 5:**

```
**Input:** s = "spo"
**Output:** "ops"
```

**Constraints:**

* `1 <= s.length <= 500`
* `s` contains only lower-case English letters.

## ac

```java
```