1370. Increasing Decreasing String

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.

1370. Increasing Decreasing String
``````struct Solution;

impl Solution {
fn sort_string(s: String) -> String {
let mut count: Vec<usize> = vec![0; 26];
let mut n = s.len();
for c in s.chars() {
count[(c as u8 - b'a') as usize] += 1;
}
let mut direction = true;
let mut res = "".to_string();
while n > 0 {
if direction {
for i in 0..26 {
if count[i] > 0 {
count[i] -= 1;
n -= 1;
res.push((b'a' + i as u8) as char);
}
}
} else {
for i in (0..26).rev() {
if count[i] > 0 {
count[i] -= 1;
n -= 1;
res.push((b'a' + i as u8) as char);
}
}
}
direction = !direction;
}
res
}
}

#[test]
fn test() {
let s = "aaaabbbbcccc".to_string();
let res = "abccbaabccba".to_string();
assert_eq!(Solution::sort_string(s), res);
let s = "rat".to_string();
let res = "art".to_string();
assert_eq!(Solution::sort_string(s), res);
let s = "leetcode".to_string();
let res = "cdelotee".to_string();
assert_eq!(Solution::sort_string(s), res);
let s = "ggggggg".to_string();
let res = "ggggggg".to_string();
assert_eq!(Solution::sort_string(s), res);
let s = "spo".to_string();
let res = "ops".to_string();
assert_eq!(Solution::sort_string(s), res);
}
``````