1370. Increasing Decreasing String

Problem Link

Description

---

You are given a string s. Reorder the string using the following algorithm:

1. Remove the smallest character from s and append it to the result.
2. Remove the smallest character from s that is greater than the last appended character, and append it to the result.
3. Repeat step 2 until no more characters can be removed.
4. Remove the largest character from s and append it to the result.
5. Remove the largest character from s that is smaller than the last appended character, and append it to the result.
6. Repeat step 5 until no more characters can be removed.
7. Repeat steps 1 through 6 until all characters from s have been removed.

If the smallest or largest character appears more than once, you may choose any occurrence to append to the result.

Return the resulting string after reordering s using 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.

 

Constraints:

• 1 <= s.length <= 500
• s consists of only lowercase English letters.

Solution

---

Python3

class Solution:
    def sortString(self, S: str) -> str:
        mp = collections.defaultdict(int)
        n = len(S)
        for s in S:
            mp[s] += 1
        
        mp = dict(sorted(mp.items()))
        res = []
        while len(res) != n:
            for key in mp:
                if mp[key]:
                    res.append(key)
                    mp[key] -= 1
 
            for key in dict(sorted(mp.items(), reverse = True)):
                if mp[key]:
                    res.append(key)
                    mp[key] -= 1
 
        
        return "".join(res)
            

C++

class Solution {
public:
    string sortString(string s, string res = "") {
        int cnt[26] = {};
        for (auto ch : s)
            ++cnt[ch - 'a'];
        
        while (res.length() != s.length()){
            for (int i = 0; i < 26; i++)
                res += string(--cnt[i] >= 0 ? 1 : 0, 'a' + i);
            
            for (int i = 25; i >= 0; i--)
                res += string(--cnt[i] >= 0 ? 1 : 0, 'a' + i);
        }
        
        return res;
            
    }
};