Guessing

This problem is to find a UNIQUE station i, such that

• SUM(gas[0 .. i-1]) - SUM(cost[0 .. i-1]) + SUM(gas[i .. end]) - SUM(cost[i .. end]) >= 0
• SUM(gas[0 .. i-1]) - SUM(cost[0 .. i-1]) < 0

SUM(gas[0 .. i-1]) - SUM(cost[0 .. i-1]) < 0 is to guarantee that the i is unique.

If SUM(gas[0 .. i-1]) - SUM(cost[0 .. i-1]) < 0 and SUM(gas[0 .. i-1]) - SUM(cost[0 .. i-1]) + SUM(gas[i .. end]) - SUM(cost[i .. end]) >= 0, that is, 0 .. i-1 are all valid stations.

Brute force

Just search through from 0 to end, find the valid i.