Increasing Decreasing String

Author: anantkaushik

1370-increasing-decreasing-string.py

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+"""
+Problem Link: https://leetcode.com/problems/increasing-decreasing-string/
+
+Given a string s. You should re-order the string using the following algorithm:
+Pick the smallest character from s and append it to the result.
+Pick the smallest character from s which is greater than the last appended character to the result 
+and append it.
+Repeat step 2 until you cannot pick more characters.
+Pick the largest character from s and append it to the result.
+Pick the largest character from s which is smaller than the last appended character to the result 
+and append it.
+Repeat step 5 until you cannot pick more characters.
+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.
+"""
+class Solution:
+    def sortString(self, s: str) -> str:
+        alphabets = [0]*26
+        for c in s:
+          alphabets[ord(c) - ord('a')] += 1
+          
+        res = []
+        flag = direction =  True
+        index = 0
+        while flag:
+          flag = False
+          while (index  < len(alphabets) and direction) or (index  >= 0 and not direction):
+            if alphabets[index] > 0:
+              res.append(chr(index + ord('a')))
+              alphabets[index] -= 1
+              flag = True
+            index += 1 if direction else -1
+          direction = False if direction else True
+          index += 1 if direction else -1
+        return "".join(res)