Gas Station

Problem n gas stations form a circle: station i provides gas[i]; driving to the next costs cost[i]. Starting with an empty tank, return the unique starting index allowing one full clockwise circuit, or -1.

Input / Output

• Input: int arrays gas, cost. Output: starting index or -1.

Constraints

• n up to 10^5; O(n) greedy expected; trying every start is O(n^2).

Example

• gas = [1,2,3,4,5], cost = [3,4,5,1,2] → 3.