feat(algorithms, greedy): largest palindromic number

DIRECTORY.md

@@ -141,6 +141,8 @@
       * [Test Gas Stations](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/greedy/gas_stations/test_gas_stations.py)
     * Jump Game
       * [Test Jump Game](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/greedy/jump_game/test_jump_game.py)
+    * Largest Palindromic Number
+      * [Test Largest Palindromic Number](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/greedy/largest_palindromic_number/test_largest_palindromic_number.py)
     * Majority Element
       * [Test Majority Element](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/greedy/majority_element/test_majority_element.py)
     * Min Arrows

algorithms/dynamic_programming/buy_sell_stock/__init__.py

@@ -26,13 +26,6 @@ def max_profit(prices: List[int]) -> int:
     return current_max_profit
 
 
-def max_profit_two_pointers(prices: List[int]) -> int:
-    """
-    Variation of max_profit using 2 pointers
-    Complexity Analysis:
-    Space: O(1), no extra memory is used
-    Time: O(n), where n is the size of the input list & we iterate through the list only once
-    """
 def max_profit_two_pointers(prices: List[int]) -> int:
     """
     Variation of max_profit using 2 pointers
@@ -46,7 +39,6 @@ def max_profit_two_pointers(prices: List[int]) -> int:
     number_of_prices = len(prices)
 
     left, right = 0, 1
-        return 0
 
     left, right = 0, 1
     current_max_profit = 0

algorithms/greedy/largest_palindromic_number/README.md

@@ -0,0 +1,68 @@
+# Largest Palindromic Number
+
+You are given a string num consisting of digits from 0 to 9. Your task is to return the largest possible palindromic
+number as a string by using some or all of the digits in num.
+
+The resulting palindromic number must not have leading zeros.
+
+> Note: You may reorder the digits freely, and you must use at least one digit from the num string.
+
+## Constraints
+
+- 1 <= `num.length` <= 1000
+- `num` consists of digits
+
+## Examples
+
+![Example 1](./images/examples/largest_palindromic_number_example_1.png)
+![Example 2](./images/examples/largest_palindromic_number_example_2.png)
+![Example 3](./images/examples/largest_palindromic_number_example_3.png)
+
+## Solution
+
+We will use the greedy pattern to solve this problem. The goal is to maximize the size of the palindrome by making
+locally optimal choices at each step. Specifically, we aim to form the largest possible palindrome by first prioritizing
+using the highest digits.
+
+The process begins by counting the frequency of each digit in the input string and storing it in a hash table. This
+allows us to determine how many times each digit appears. Starting with the highest digit, i.e., 9, and working down to
+the lowest, i.e., 0, we try to use the possible pairs of each digit to form the first half of the palindrome. This
+ensures that the most significant positions in the palindrome are filled with the largest digits. Out of the leftover
+single digits, the highest possible digit can be used as the middle digit to further enhance the size of the palindrome.
+
+Finally, the palindrome is completed by appending the reverse of the first half to itself, with the middle digit in
+between, if applicable. This greedy strategy works effectively because it ensures that each decision made is the best
+possible choice at that moment, leading to an overall optimal solution. 
+
+Let’s review the algorithm to reach the solution:
+
+- Initialize the frequency counter occurrences to count the frequency of each digit in the input string num.
+- We also initialize the first_half array to note down the first part of the palindrome and the middle string to track
+  the middle element of the palindrome.
+- Traverse digits from 9 to 0 and for each digit do the following:
+  - Check if its pairs can be made by looking at its frequency. If yes, then add it to the first_half array. 
+  - If applicable, check for the leading zeros and avoid them by explicitly setting their occurrence to 1. 
+  - Otherwise, check if it can be the middle element of the palindrome. Following the greedy approach, ensures that a
+    larger number is selected to be the middle element among the elements occurring once.
+
+- Once we have processed all the elements of the num array, we join the first_half, middle, and reverse of the first_half. 
+- Finally, we return this palindromic number, the largest that can be generated using the given number.
+
+![Solution 1](./images/solutions/largest_palindromic_number_solution_1.png)
+![Solution 2](./images/solutions/largest_palindromic_number_solution_2.png)
+![Solution 3](./images/solutions/largest_palindromic_number_solution_3.png)
+![Solution 4](./images/solutions/largest_palindromic_number_solution_4.png)
+![Solution 5](./images/solutions/largest_palindromic_number_solution_5.png)
+![Solution 6](./images/solutions/largest_palindromic_number_solution_6.png)
+![Solution 7](./images/solutions/largest_palindromic_number_solution_7.png)
+![Solution 8](./images/solutions/largest_palindromic_number_solution_8.png)
+![Solution 9](./images/solutions/largest_palindromic_number_solution_9.png)
+
+### Time Complexity
+
+The time complexity of the solution is O(n), where n is the length of the num string.
+
+### Space Complexity
+
+The space complexity of the solution is O(n). In the worst case, first_half could store up to n/2 digits if all digits
+are the same. Therefore, the space for first_half is O(n).

algorithms/greedy/largest_palindromic_number/__init__.py

@@ -0,0 +1,65 @@
+from collections import Counter
+
+
+def largest_palindromic_number(num: str) -> str:
+    # Count the frequency of each digit in the input string
+    occureneces = Counter(num)
+
+    # first half and the middle of the palindrome
+    first_half = []
+    middle = ""
+
+    for digit in range(9, -1, -1):
+        digit_char = str(digit)
+
+        if digit_char in occureneces:
+            digit_count = occureneces[digit_char]
+
+            # num of pairs of this digit that can be used
+            num_pairs = digit_count // 2
+
+            # If pairs available, add them to the first half
+            if num_pairs:
+                # Avoiding leading zeros
+                if not first_half and not digit:
+                    occureneces["0"] = 1
+                else:
+                    first_half.append(digit_char * num_pairs)
+
+            # Checking for a middle element
+            if digit_count % 2 and not middle:
+                middle = digit_char
+
+    # If all elements are '0'
+    if not middle and not first_half:
+        return "0"
+
+    # Returning the full palindrome
+    return "".join(first_half + [middle] + first_half[::-1])
+
+
+def largest_palindromic_number_v2(num: str) -> str:
+    # Count how many times each digit appears
+    digit_counter = Counter(num)
+
+    result = ""
+    # Find the middle digit by counting down from 9 to 0 while decrementing the count of a digit that appears an odd
+    # number of times
+    for digit in range(9, 0, -1):
+        digit_str = str(digit)
+        if digit_counter[digit_str] % 2 == 1:
+            result = digit_str
+            # decrement the count of this digit
+            digit_counter[digit_str] -= 1
+            break
+
+    # Build symmetric pairs by iterating from 0 to 9 for each digit with remaining count
+    for number in range(10):
+        number_str = str(number)
+        if digit_counter[number_str] > 0:
+            # Divide by two to get number of pairs
+            digit_counter[number_str] //= 2
+            temp = digit_counter[number_str] * number_str
+            result = f"{temp}{result}{temp}"
+
+    return result.strip("0") or "0"

algorithms/greedy/largest_palindromic_number/test_largest_palindromic_number.py

@@ -0,0 +1,32 @@
+import unittest
+from parameterized import parameterized
+from algorithms.greedy.largest_palindromic_number import (
+    largest_palindromic_number,
+    largest_palindromic_number_v2,
+)
+
+LARGEST_PALINDROMIC_NUMBER_TEST_CASES = [
+    ("2012", "212"),
+    ("000001", "1"),
+    ("111222333", "3213123"),
+    ("000000", "0"),
+    ("123456789123456789", "987654321123456789"),
+    ("444947137", "7449447"),
+    ("00009", "9"),
+]
+
+
+class LargestPalindromicNumberTestCase(unittest.TestCase):
+    @parameterized.expand(LARGEST_PALINDROMIC_NUMBER_TEST_CASES)
+    def test_largest_palindromic_number(self, num: str, expected: str):
+        actual = largest_palindromic_number(num)
+        self.assertEqual(expected, actual)
+
+    @parameterized.expand(LARGEST_PALINDROMIC_NUMBER_TEST_CASES)
+    def test_largest_palindromic_number_v2(self, num: str, expected: str):
+        actual = largest_palindromic_number_v2(num)
+        self.assertEqual(expected, actual)
+
+
+if __name__ == "__main__":
+    unittest.main()