LeetCode #134 — MEDIUM

Gas Station

Move from brute-force thinking to an efficient approach using array strategy.

The Problem

Problem Statement

There are n gas stations along a circular route, where the amount of gas at the i^th station is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from the i^th station to its next (i + 1)^th station. You begin the journey with an empty tank at one of the gas stations.

Given two integer arrays gas and cost, return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1. If there exists a solution, it is guaranteed to be unique.

Example 1:

Input: gas = [1,2,3,4,5], cost = [3,4,5,1,2]
Output: 3
Explanation:
Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 4. Your tank = 4 - 1 + 5 = 8
Travel to station 0. Your tank = 8 - 2 + 1 = 7
Travel to station 1. Your tank = 7 - 3 + 2 = 6
Travel to station 2. Your tank = 6 - 4 + 3 = 5
Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
Therefore, return 3 as the starting index.

Example 2:

Input: gas = [2,3,4], cost = [3,4,3]
Output: -1
Explanation:
You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 0. Your tank = 4 - 3 + 2 = 3
Travel to station 1. Your tank = 3 - 3 + 3 = 3
You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
Therefore, you can't travel around the circuit once no matter where you start.

Constraints:

n == gas.length == cost.length
1 <= n <= 10⁵
0 <= gas[i], cost[i] <= 10⁴
The input is generated such that the answer is unique.

Step 01

Brute Force Baseline

Problem summary: There are n gas stations along a circular route, where the amount of gas at the ith station is gas[i]. You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from the ith station to its next (i + 1)th station. You begin the journey with an empty tank at one of the gas stations. Given two integer arrays gas and cost, return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1. If there exists a solution, it is guaranteed to be unique.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array · Greedy

Example 1

[1,2,3,4,5]
[3,4,5,1,2]

Example 2

[2,3,4]
[3,4,3]

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #134: Gas Station
class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int n = gas.length;
        int i = n - 1, j = n - 1;
        int cnt = 0, s = 0;
        while (cnt < n) {
            s += gas[j] - cost[j];
            ++cnt;
            j = (j + 1) % n;
            while (s < 0 && cnt < n) {
                --i;
                s += gas[i] - cost[i];
                ++cnt;
            }
        }
        return s < 0 ? -1 : i;
    }
}

// Accepted solution for LeetCode #134: Gas Station
func canCompleteCircuit(gas []int, cost []int) int {
	n := len(gas)
	i, j := n-1, n-1
	cnt, s := 0, 0
	for cnt < n {
		s += gas[j] - cost[j]
		cnt++
		j = (j + 1) % n
		for s < 0 && cnt < n {
			i--
			s += gas[i] - cost[i]
			cnt++
		}
	}
	if s < 0 {
		return -1
	}
	return i
}

# Accepted solution for LeetCode #134: Gas Station
class Solution:
    def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
        n = len(gas)
        i = j = n - 1
        cnt = s = 0
        while cnt < n:
            s += gas[j] - cost[j]
            cnt += 1
            j = (j + 1) % n
            while s < 0 and cnt < n:
                i -= 1
                s += gas[i] - cost[i]
                cnt += 1
        return -1 if s < 0 else i

// Accepted solution for LeetCode #134: Gas Station
struct Solution;

impl Solution {
    fn can_complete_circuit(gas: Vec<i32>, cost: Vec<i32>) -> i32 {
        let n = gas.len();

        let mut total = 0;
        let mut cur = 0;
        let mut idx = 0;

        for i in 0..n {
            total += gas[i] - cost[i];
            cur += gas[i] - cost[i];

            if cur < 0 {
                idx = i + 1;
                cur = 0;
            }
        }

        if total >= 0 {
            idx as i32
        } else {
            -1
        }
    }
}

#[test]
fn test() {
    let gas = vec![1, 2, 3, 4, 5];
    let cost = vec![3, 4, 5, 1, 2];
    let res = 3;
    assert_eq!(Solution::can_complete_circuit(gas, cost), res);
}

// Accepted solution for LeetCode #134: Gas Station
function canCompleteCircuit(gas: number[], cost: number[]): number {
    const n = gas.length;
    let i = n - 1;
    let j = n - 1;
    let s = 0;
    let cnt = 0;
    while (cnt < n) {
        s += gas[j] - cost[j];
        ++cnt;
        j = (j + 1) % n;
        while (s < 0 && cnt < n) {
            --i;
            s += gas[i] - cost[i];
            ++cnt;
        }
    }
    return s < 0 ? -1 : i;
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n log n)

Space

O(1)

Approach Breakdown

EXHAUSTIVE

O(2ⁿ) time

O(n) space

Try every possible combination of choices. With n items each having two states (include/exclude), the search space is 2ⁿ. Evaluating each combination takes O(n), giving O(n × 2ⁿ). The recursion stack or subset storage uses O(n) space.

GREEDY

O(n log n) time

O(1) space

Greedy algorithms typically sort the input (O(n log n)) then make a single pass (O(n)). The sort dominates. If the input is already sorted or the greedy choice can be computed without sorting, time drops to O(n). Proving greedy correctness (exchange argument) is harder than the implementation.

Shortcut: Sort + single pass → O(n log n). If no sort needed → O(n). The hard part is proving it works.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.

Using greedy without proof

Wrong move: Locally optimal choices may fail globally.

Usually fails on: Counterexamples appear on crafted input orderings.

Fix: Verify with exchange argument or monotonic objective before committing.