feat: add gas_station (#50)

Author: wislertt

Makefile

@@ -1,5 +1,5 @@
 PYTHON_VERSION = 3.13
-PROBLEM ?= house_robber
+PROBLEM ?= gas_station
 FORCE ?= 0
 COMMA := ,
 

leetcode/gas_station/README.md

@@ -0,0 +1,53 @@
+# Gas Station
+
+**Difficulty:** Medium
+**Topics:** Array, Greedy
+**Tags:** grind
+
+**LeetCode:** [Problem 134](https://leetcode.com/problems/gas-station/description/)
+
+## Problem Description
+
+There are `n` gas stations along a circular route, where the amount of gas at the `ith` station is `gas[i]`.
+
+You have a car with an unlimited gas tank and it costs `cost[i]` of gas to travel from the `ith` station to its next `(i + 1)th` station. You begin the journey with an empty tank at one of the gas stations.
+
+Given two integer arrays `gas` and `cost`, return _the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return_ `-1`. If there exists a solution, it is **guaranteed** to be **unique**.
+
+## Examples
+
+### Example 1:
+
+```
+Input: gas = [1,2,3,4,5], cost = [3,4,5,1,2]
+Output: 3
+Explanation:
+Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
+Travel to station 4. Your tank = 4 - 1 + 5 = 8
+Travel to station 0. Your tank = 8 - 2 + 1 = 7
+Travel to station 1. Your tank = 7 - 3 + 2 = 6
+Travel to station 2. Your tank = 6 - 4 + 3 = 5
+Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
+Therefore, return 3 as the starting index.
+```
+
+### Example 2:
+
+```
+Input: gas = [2,3,4], cost = [3,4,3]
+Output: -1
+Explanation:
+You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
+Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
+Travel to station 0. Your tank = 4 - 3 + 2 = 3
+Travel to station 1. Your tank = 3 - 3 + 3 = 3
+You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
+Therefore, you can't travel around the circuit once no matter where you start.
+```
+
+## Constraints
+
+- `n == gas.length == cost.length`
+- `1 <= n <= 10^5`
+- `0 <= gas[i], cost[i] <= 10^4`
+- The input is generated such that the answer is unique.

leetcode/gas_station/__init__.py

@@ -0,0 +1,8 @@
+def run_can_complete_circuit(solution_class: type, gas: list[int], cost: list[int]):
+    implementation = solution_class()
+    return implementation.can_complete_circuit(gas, cost)
+
+
+def assert_can_complete_circuit(result: int, expected: int) -> bool:
+    assert result == expected
+    return True

leetcode/gas_station/helpers.py

@@ -0,0 +1,30 @@
+# ---
+# jupyter:
+#   jupytext:
+#     text_representation:
+#       extension: .py
+#       format_name: percent
+#       format_version: '1.3'
+#       jupytext_version: 1.17.3
+#   kernelspec:
+#     display_name: leetcode-py-py3.13
+#     language: python
+#     name: python3
+# ---
+
+# %%
+from helpers import assert_can_complete_circuit, run_can_complete_circuit
+from solution import Solution
+
+# %%
+# Example test case
+gas = [1, 2, 3, 4, 5]
+cost = [3, 4, 5, 1, 2]
+expected = 3
+
+# %%
+result = run_can_complete_circuit(Solution, gas, cost)
+result
+
+# %%
+assert_can_complete_circuit(result, expected)

leetcode/gas_station/playground.py

@@ -0,0 +1,37 @@
+class Solution:
+    """
+    Gas Station Circuit - Greedy Approach
+
+    Visual Example: gas=[1,2,3,4,5], cost=[3,4,5,1,2]
+
+         Station:  0   1   2   3   4
+                  ┌─┐ ┌─┐ ┌─┐ ┌─┐ ┌─┐
+         Gas:     │1│ │2│ │3│ │4│ │5│
+                  └─┘ └─┘ └─┘ └─┘ └─┘
+         Cost:     3   4   5   1   2
+                   ↓   ↓   ↓   ↓   ↓
+         Net:     -2  -2  -2  +3  +3
+
+    Algorithm trace:
+    i=0: tank=0+(-2)=-2 < 0 → reset tank=0, start=1
+    i=1: tank=0+(-2)=-2 < 0 → reset tank=0, start=2
+    i=2: tank=0+(-2)=-2 < 0 → reset tank=0, start=3
+    i=3: tank=0+(+3)=+3 ≥ 0 → continue
+    i=4: tank=3+(+3)=+6 ≥ 0 → return start=3
+
+    Key insight: If total_gas ≥ total_cost, greedy start position works!
+    """
+
+    # Time: O(n)
+    # Space: O(1)
+    def can_complete_circuit(self, gas: list[int], cost: list[int]) -> int:
+        if sum(gas) < sum(cost):
+            return -1
+
+        tank = start = 0
+        for i in range(len(gas)):
+            tank += gas[i] - cost[i]
+            if tank < 0:
+                tank = 0
+                start = i + 1
+        return start

leetcode/gas_station/solution.py

@@ -0,0 +1,33 @@
+import pytest
+
+from leetcode_py import logged_test
+
+from .helpers import assert_can_complete_circuit, run_can_complete_circuit
+from .solution import Solution
+
+
+class TestGasStation:
+    def setup_method(self):
+        self.solution = Solution()
+
+    @logged_test
+    @pytest.mark.parametrize(
+        "gas, cost, expected",
+        [
+            ([1, 2, 3, 4, 5], [3, 4, 5, 1, 2], 3),
+            ([2, 3, 4], [3, 4, 3], -1),
+            ([1, 2], [2, 1], 1),
+            ([5], [4], 0),
+            ([2], [2], 0),
+            ([1, 2, 3], [3, 3, 3], -1),
+            ([3, 1, 1], [1, 2, 2], 0),
+            ([5, 1, 2, 3, 4], [4, 4, 1, 5, 1], 4),
+            ([1, 2, 3, 4, 5, 5, 70], [2, 3, 4, 3, 9, 6, 2], 6),
+            ([4, 5, 2, 6, 5, 3], [3, 2, 7, 3, 2, 9], -1),
+            ([6, 1, 4, 3, 5], [3, 8, 2, 4, 2], 2),
+            ([2, 3, 4, 5], [3, 4, 5, 6], -1),
+        ],
+    )
+    def test_can_complete_circuit(self, gas: list[int], cost: list[int], expected: int):
+        result = run_can_complete_circuit(Solution, gas, cost)
+        assert_can_complete_circuit(result, expected)

leetcode/gas_station/test_solution.py

@@ -0,0 +1,62 @@
+{
+    "problem_name": "gas_station",
+    "solution_class_name": "Solution",
+    "problem_number": "134",
+    "problem_title": "Gas Station",
+    "difficulty": "Medium",
+    "topics": "Array, Greedy",
+    "readme_description": "There are `n` gas stations along a circular route, where the amount of gas at the `ith` station is `gas[i]`.\n\nYou have a car with an unlimited gas tank and it costs `cost[i]` of gas to travel from the `ith` station to its next `(i + 1)th` station. You begin the journey with an empty tank at one of the gas stations.\n\nGiven two integer arrays `gas` and `cost`, return *the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return* `-1`. If there exists a solution, it is **guaranteed** to be **unique**.",
+    "_readme_examples": {
+        "list": [
+            {
+                "content": "```\nInput: gas = [1,2,3,4,5], cost = [3,4,5,1,2]\nOutput: 3\nExplanation:\nStart at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4\nTravel to station 4. Your tank = 4 - 1 + 5 = 8\nTravel to station 0. Your tank = 8 - 2 + 1 = 7\nTravel to station 1. Your tank = 7 - 3 + 2 = 6\nTravel to station 2. Your tank = 6 - 4 + 3 = 5\nTravel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.\nTherefore, return 3 as the starting index.\n```"
+            },
+            {
+                "content": "```\nInput: gas = [2,3,4], cost = [3,4,3]\nOutput: -1\nExplanation:\nYou can't start at station 0 or 1, as there is not enough gas to travel to the next station.\nLet's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4\nTravel to station 0. Your tank = 4 - 3 + 2 = 3\nTravel to station 1. Your tank = 3 - 3 + 3 = 3\nYou cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.\nTherefore, you can't travel around the circuit once no matter where you start.\n```"
+            }
+        ]
+    },
+    "readme_constraints": "- `n == gas.length == cost.length`\n- `1 <= n <= 10^5`\n- `0 <= gas[i], cost[i] <= 10^4`\n- The input is generated such that the answer is unique.",
+    "helpers_imports": "",
+    "helpers_content": "",
+    "helpers_run_name": "can_complete_circuit",
+    "helpers_run_signature": "(solution_class: type, gas: list[int], cost: list[int])",
+    "helpers_run_body": "    implementation = solution_class()\n    return implementation.can_complete_circuit(gas, cost)",
+    "helpers_assert_name": "can_complete_circuit",
+    "helpers_assert_signature": "(result: int, expected: int) -> bool",
+    "helpers_assert_body": "    assert result == expected\n    return True",
+    "solution_imports": "",
+    "solution_contents": "",
+    "solution_class_content": "",
+    "test_imports": "import pytest\nfrom leetcode_py import logged_test\nfrom .helpers import assert_can_complete_circuit, run_can_complete_circuit\nfrom .solution import Solution",
+    "test_content": "",
+    "test_class_name": "GasStation",
+    "test_class_content": "    def setup_method(self):\n        self.solution = Solution()",
+    "_solution_methods": {
+        "list": [
+            {
+                "name": "can_complete_circuit",
+                "signature": "(self, gas: list[int], cost: list[int]) -> int",
+                "body": "        # TODO: Implement can_complete_circuit\n        return -1"
+            }
+        ]
+    },
+    "_test_helper_methods": {
+        "list": [{ "name": "setup_method", "parameters": "", "body": "self.solution = Solution()" }]
+    },
+    "_test_methods": {
+        "list": [
+            {
+                "name": "test_can_complete_circuit",
+                "signature": "(self, gas: list[int], cost: list[int], expected: int)",
+                "parametrize": "gas, cost, expected",
+                "test_cases": "[([1,2,3,4,5], [3,4,5,1,2], 3), ([2,3,4], [3,4,3], -1), ([1,2], [2,1], 1), ([5], [4], 0), ([2], [2], 0), ([1,2,3], [3,3,3], -1), ([3,1,1], [1,2,2], 0), ([5,1,2,3,4], [4,4,1,5,1], 4), ([1,2,3,4,5,5,70], [2,3,4,3,9,6,2], 6), ([4,5,2,6,5,3], [3,2,7,3,2,9], -1), ([6,1,4,3,5], [3,8,2,4,2], 2), ([2,3,4,5], [3,4,5,6], -1)]",
+                "body": "        result = run_can_complete_circuit(Solution, gas, cost)\n        assert_can_complete_circuit(result, expected)"
+            }
+        ]
+    },
+    "playground_imports": "from helpers import run_can_complete_circuit, assert_can_complete_circuit\nfrom solution import Solution",
+    "playground_setup": "# Example test case\ngas = [1,2,3,4,5]\ncost = [3,4,5,1,2]\nexpected = 3",
+    "playground_run": "result = run_can_complete_circuit(Solution, gas, cost)\nresult",
+    "playground_assert": "assert_can_complete_circuit(result, expected)"
+}

leetcode_py/cli/resources/leetcode/json/problems/gas_station.json

@@ -78,7 +78,7 @@
         "zero_one_matrix",
     ],
 
-    grind: [{ tag: "grind-75" }, "daily_temperatures", "house_robber"],
+    grind: [{ tag: "grind-75" }, "daily_temperatures", "gas_station", "house_robber"],
 
     // Test tag for development and testing
     test: ["binary_search", "two_sum", "valid_palindrome"],