Time: 3 ms (98.34%), Space: 17.9 MB (61.48%) - LeetHub

0038-count-and-say/0038-count-and-say.py

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+class Solution:
+    def countAndSay(self, n: int) -> str:
+        """
+        Complexities:
+        Time:  O(n2^n)
+        Space: O()
+        """
+
+        # O(len(s)) where len(s) ~ O(2^n) 
+        def RLE(s: str) -> str:
+            rle = ""
+            checkChar = s[0]
+            charCount = 0
+            for i, l in enumerate(s):
+                if checkChar == l:
+                    charCount += 1
+                else:
+                    rle += f"{charCount}{checkChar}"
+                    checkChar = l
+                    charCount = 1
+
+            if checkChar:
+                rle += f"{charCount}{checkChar}"
+
+            return rle
+
+        lastSol = "1"
+        # Iteratively call RLE on lastSol (equiv. to countAndSay(n-1))
+        # - O(n2^n) = n * O(2^n)
+        for i in range(2, n + 1):
+            lastSol = RLE(lastSol)
+
+        return lastSol