Practice/Google/Leetcode 1279. Traffic Light Controlled Intersection

Leetcode 1279. Traffic Light Controlled Intersection

CodingMust

Problem

You need to simulate a four-way intersection controlled by a traffic light. The intersection allows traffic in two perpendicular directions:

• North-South (directions 0 and 2): Cars traveling North or South
• East-West (directions 1 and 3): Cars traveling East or West

The traffic light alternates between these two states. Initially, the light allows North-South traffic. The light switches to allow the perpendicular direction only when:

1. There are no more cars in the current direction's queue, AND
2. There is at least one car waiting in the perpendicular direction

Cars arrive at the intersection at specific times. Each car has an arrival time and requires a certain amount of time to cross the intersection. Cars from the same direction cross in the order they arrive (FIFO). A car can only start crossing when:

• The light allows its direction
• No other car from its direction group is currently crossing
• The car has arrived at the intersection

Your task is to determine when the last car finishes crossing the intersection.

Requirements

1. Implement a traffic light controller that switches between North-South and East-West traffic
2. Process cars in FIFO order within their direction group (North-South or East-West)
3. The light only switches when the current direction has no cars waiting or crossing AND the perpendicular direction has cars waiting
4. Return the time at which the final car completes crossing

Constraints

• 1 ≤ arrivals.length ≤ 100
• Each arrival is [car_id, direction, arrival_time, travel_time]
• 0 ≤ direction ≤ 3 (0=North, 1=East, 2=South, 3=West)
• 0 ≤ arrival_time ≤ 10,000
• 1 ≤ travel_time ≤ 100
• All car_id values are unique
• arrivals may not be sorted by arrival time

Examples

Example 1:

` Input: arrivals = [[1, 0, 0, 2], [2, 0, 1, 1]] Output: 3 Explanation:

• Car 1 arrives at time 0, going North, needs 2 seconds
• Car 2 arrives at time 1, going North, needs 1 second
• Light starts allowing North-South traffic
• Car 1 crosses from time 0 to 2
• Car 2 crosses from time 2 to 3
• Last car finishes at time 3 `

Example 2:

` Input: arrivals = [[1, 0, 0, 5], [2, 1, 2, 3]] Output: 8 Explanation:

• Car 1 (North) arrives at 0, needs 5 seconds
• Car 2 (East) arrives at 2, needs 3 seconds
• Light starts allowing North-South
• Car 1 crosses from time 0 to 5
• At time 5, no North-South cars remain, light switches to East-West
• Car 2 crosses from time 5 to 8
• Last car finishes at time 8 `

Example 3:

` Input: arrivals = [[1, 0, 5, 2], [2, 1, 3, 4]] Output: 9 Explanation:

• Car 2 (East) arrives at 3, needs 4 seconds
• Car 1 (North) arrives at 5, needs 2 seconds
• Light starts allowing North-South, but no cars yet
• Car 2 arrives at 3 for East-West direction, but must wait
• Car 1 arrives at 5, can start immediately (North-South allowed)
• Car 1 crosses from 5 to 7
• Light switches to East-West at time 7
• Car 2 has been waiting since time 3, crosses from 7 to 11... wait no
• Actually car 2 waits until light switches, crosses 7 to 11... no that's 4 seconds
• Car 2 crosses from time 7 to 7+4=11... but answer says 9
• Let me reconsider: if light starts NS, and first car is EW at time 3, light CAN switch immediately since no NS cars are present. Car 2 crosses 3-7. Car 1 arrives at 5 (during car 2's crossing), must wait. At time 7, light switches to NS. Car 1 crosses 7-9. `