test: migrate LeetCode 2351 to new directory structure with tests and docs

README.md

@@ -41,11 +41,11 @@ Each problem follows this structure:
 
 ## Solutions
 
-📊 **Statistics**: 89 problems solved (migrated to new structure)
+📊 **Statistics**: 90 problems solved (migrated to new structure)
 
 | Difficulty | Count |
 |------------|-------|
-| Easy       | 61    |
+| Easy       | 62    |
 | Medium     | 26    |
 | Hard       | 2     |
 

docs/EASY.md

@@ -1,6 +1,6 @@
 # Easy Problems
 
-Total: 61 problems solved
+Total: 62 problems solved
 
 ## Solutions
 
@@ -60,6 +60,7 @@ Total: 61 problems solved
 | 2185 | [Counting Words With a Given Prefix](https://leetcode.com/problems/counting-words-with-a-given-prefix/) | [solution.py](../problems/easy/counting_words_with_a_given_prefix_2185/solution.py) | [test_solution.py](../problems/easy/counting_words_with_a_given_prefix_2185/test_solution.py) |
 | 2215 | [Find the Difference of Two Arrays](https://leetcode.com/problems/find-the-difference-of-two-arrays/) | [solution.py](../problems/easy/find_the_difference_of_two_arrays_2215/solution.py) | [test_solution.py](../problems/easy/find_the_difference_of_two_arrays_2215/test_solution.py) |
 | 2315 | [Count Asterisks](https://leetcode.com/problems/count-asterisks/) | [solution.py](../problems/easy/count_asterisks_2315/solution.py) | [test_solution.py](../problems/easy/count_asterisks_2315/test_solution.py) |
+| 2351 | [First Letter to Appear Twice](https://leetcode.com/problems/first-letter-to-appear-twice/) | [solution.py](../problems/easy/first_letter_to_appear_twice_2351/solution.py) | [test_solution.py](../problems/easy/first_letter_to_appear_twice_2351/test_solution.py) |
 | 2469 | [Convert the Temperature](https://leetcode.com/problems/convert-the-temperature/) | [solution.py](../problems/easy/convert_the_temperature_2469/solution.py) | [test_solution.py](../problems/easy/convert_the_temperature_2469/test_solution.py) |
 | 2710 | [Remove Trailing Zeros From a String](https://leetcode.com/problems/remove-trailing-zeros-from-a-string/) | [solution.py](../problems/easy/remove_trailing_zeros_from_a_string_2710/solution.py) | [test_solution.py](../problems/easy/remove_trailing_zeros_from_a_string_2710/test_solution.py) |
 | 2798 | [Number of Employees Who Met the Target](https://leetcode.com/problems/number-of-employees-who-met-the-target/) | [solution.py](../problems/easy/number_of_employees_who_met_the_target_2798/solution.py) | [test_solution.py](../problems/easy/number_of_employees_who_met_the_target_2798/test_solution.py) |

problems/easy/first_letter_to_appear_twice_2351/solution.py

@@ -0,0 +1,45 @@
+# Tags: Hash Table, String
+class Solution:
+    def repeated_character(self, s: str) -> str:
+        """
+        Return the first letter whose second occurrence appears earliest in s.
+
+        Args:
+            s: Input string of lowercase English letters in which at least one
+                letter appears twice.
+
+        Returns:
+            The first letter to appear twice, i.e. the letter whose second
+            occurrence has the smallest index.
+
+        Time complexity: O(n)
+        - n is the length of s, k is the number of distinct letters (k <= n)
+        - The for loop visits each character at most once, so at most n iterations
+        - Each iteration does one set membership test (`c in seen`) and at most
+          one set insertion (`seen.add(c)`), both O(1) on average
+        - Cost per iteration is O(1), so the total is O(n) * O(1) = O(n)
+        - Best case: the first repeat is found early (e.g. "aa") and the loop
+          returns after 2 iterations: O(1)
+        - Worst case: the repeat is at the very end, so the loop runs through
+          all n characters: O(n)
+        - Hash collisions could degrade set operations to O(k) in theory, but
+          with single-character keys this is not a practical concern
+        - Under the problem constraints (lowercase letters only), the loop runs
+          at most 27 iterations (26 distinct letters + 1 repeat), because by the
+          pigeonhole principle the 27th character must repeat an earlier one,
+          so the time is effectively O(1)
+
+        Space complexity: O(k)
+        - The set `seen` stores each distinct letter at most once: O(k)
+        - Because the loop returns on the first repeat, `seen` never holds more
+          than min(n, alphabet size) letters
+        - No recursion, slicing or copying, so there is no extra call stack or
+          temporary storage; the returned single character is O(1)
+        - With a fixed alphabet of 26 lowercase letters, k <= 26, so the
+          auxiliary space is O(1)
+        """
+        seen = set()
+        for c in s:
+            if c in seen:
+                return c
+            seen.add(c)

problems/easy/first_letter_to_appear_twice_2351/test_solution.py

@@ -0,0 +1,14 @@
+import pytest
+from tests.base_test import BaseTestSolution
+from .solution import Solution
+
+
+class TestSolution(BaseTestSolution):
+    solution = Solution()
+
+    @pytest.mark.parametrize("method_name, s, expected, timeout", [
+        ('repeated_character', "abccbaacz", "c", BaseTestSolution.DEFAULT_TIMEOUT),
+        ('repeated_character', "abcdd", "d", BaseTestSolution.DEFAULT_TIMEOUT),
+    ])
+    def test_repeated_character(self, method_name, s, expected, timeout):
+        self._run_test(self.solution, method_name, (s,), expected, timeout)