feat: add explanation

Author: wislertt

leetcode/gas_station/solution.py

@@ -1,4 +1,26 @@
 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)