#125 Medium Time O(n) Space O(1) Animated

Gas Station

Find a starting gas station to complete the circuit.

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1 Predict
2 Trace
3 Explain
4 Implement

Pattern Greedy choice with a local proof

Commit to the locally best safe choice only after identifying why it cannot block an optimal answer.

Approach: Greedy

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Core Insight

Commit to the locally best safe choice only after identifying why it cannot block an optimal answer.

Common Pitfall

Sort or define the selection order before applying the greedy rule.

Description

Find a starting gas station to complete the circuit.

Constraints

Time Complexity: O(n)
Space Complexity: O(1)

Solution — Greedy

TimeO(n) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

Order candidates by the greedy criterion.
Inspect the next candidate.
Accept it only when it preserves feasibility.
Return the accumulated result.

def canCompleteCircuit(gas, cost):
    if sum(gas) < sum(cost):
        return -1
        
    total = 0
    res = 0
    for i in range(len(gas)):
        total += (gas[i] - cost[i])
        if total < 0:
            total = 0
            res = i + 1
            
    return res

Jump Game II

Hand of Straights

Gas Station

Steps

Description

Constraints

Tags

Solution — Greedy