# LeetCode 134: Gas Station — Step-by-Step Visual Trace

**Medium** — Greedy | Array | Simulation

## The Problem

Find the starting gas station index from which you can complete a circular trip, where each station has gas to collect and cost to travel to the next station.

## Approach

Use a greedy single-pass algorithm that tracks total gas deficit and resets the starting position whenever current gas becomes negative. If total gas is non-negative, a solution exists and the last reset position is the answer.

**Time:** O(n) · **Space:** O(1)

## Code

```python
class Solution:
    def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
        total_gas = 0
        current_gas = 0
        start_station = 0

        for i in range(len(gas)):
            total_gas += gas[i] - cost[i]
            current_gas += gas[i] - cost[i]

            if current_gas < 0:
                start_station = i + 1
                current_gas = 0

        return start_station if total_gas >= 0 else -1
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0134_gas-station)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0134_gas-station) and step through every line.

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