Problem Statement
There are n gas stations along a circular route. Given gas and cost arrays, return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1.
Approach
If total gas < total cost, impossible. Otherwise, a solution exists. Iterate through. Track `currentTank`. If `currentTank` drops below 0, reset start point to i+1 and `currentTank` to 0. The valid start point survives.
Time & Space Complexity
Time complexity is O(n). Space complexity is O(1).