2211-CountCollisionsOnARoad.go

package main

// 2211. Count Collisions on a Road
// There are n cars on an infinitely long road. 
// The cars are numbered from 0 to n - 1 from left to right and each car is present at a unique point.

// You are given a 0-indexed string directions of length n. 
// directions[i] can be either 'L', 'R', or 'S' denoting whether the ith car is moving towards the left, towards the right, or staying at its current point respectively. 
// Each moving car has the same speed.

// The number of collisions can be calculated as follows:
//     When two cars moving in opposite directions collide with each other, the number of collisions increases by 2.
//     When a moving car collides with a stationary car, the number of collisions increases by 1.

// After a collision, the cars involved can no longer move and will stay at the point where they collided. 
// Other than that, cars cannot change their state or direction of motion.

// Return the total number of collisions that will happen on the road.

// Example 1:
// Input: directions = "RLRSLL"
// Output: 5
// Explanation:
// The collisions that will happen on the road are:
// - Cars 0 and 1 will collide with each other. Since they are moving in opposite directions, the number of collisions becomes 0 + 2 = 2.
// - Cars 2 and 3 will collide with each other. Since car 3 is stationary, the number of collisions becomes 2 + 1 = 3.
// - Cars 3 and 4 will collide with each other. Since car 3 is stationary, the number of collisions becomes 3 + 1 = 4.
// - Cars 4 and 5 will collide with each other. After car 4 collides with car 3, it will stay at the point of collision and get hit by car 5. The number of collisions becomes 4 + 1 = 5.
// Thus, the total number of collisions that will happen on the road is 5. 

// Example 2:
// Input: directions = "LLRR"
// Output: 0
// Explanation:
// No cars will collide with each other. Thus, the total number of collisions that will happen on the road is 0.

// Constraints:
//     1 <= directions.length <= 10^5
//     directions[i] is either 'L', 'R', or 'S'.

import "fmt"
import "strings"

func countCollisions(directions string) int {
    res, rcount, scount := 0, 0, 0
    for _, v := range directions {
        if v == 'R' {
            rcount++
            continue
        }
        if v == 'L' && rcount + scount == 0 { continue } // no collision
        if v == 'L' { // SL
            res++
        }
        res += rcount // RS
        rcount = 0
        scount |= 1
    }
    return res
}

func countCollisions1(dir string) int {
    dir = strings.TrimLeft(dir, "L")
    dir = strings.TrimRight(dir, "R")
    return len(dir) - strings.Count(dir, "S")
}

func main() {
    // Example 1:
    // Input: directions = "RLRSLL"
    // Output: 5
    // Explanation:
    // The collisions that will happen on the road are:
    // - Cars 0 and 1 will collide with each other. Since they are moving in opposite directions, the number of collisions becomes 0 + 2 = 2.
    // - Cars 2 and 3 will collide with each other. Since car 3 is stationary, the number of collisions becomes 2 + 1 = 3.
    // - Cars 3 and 4 will collide with each other. Since car 3 is stationary, the number of collisions becomes 3 + 1 = 4.
    // - Cars 4 and 5 will collide with each other. After car 4 collides with car 3, it will stay at the point of collision and get hit by car 5. The number of collisions becomes 4 + 1 = 5.
    // Thus, the total number of collisions that will happen on the road is 5. 
    fmt.Println(countCollisions("RLRSLL")) // 5
    // Example 2:
    // Input: directions = "LLRR"
    // Output: 0
    // Explanation:
    // No cars will collide with each other. Thus, the total number of collisions that will happen on the road is 0.
    fmt.Println(countCollisions("LLRR")) // 0

    fmt.Println(countCollisions("LLLLLLLLL")) // 0
    fmt.Println(countCollisions("RRRRRRRRR")) // 0
    fmt.Println(countCollisions("SSSSSSSSS")) // 0
    fmt.Println(countCollisions("LLLRRRSS")) // 3
    fmt.Println(countCollisions("LRSLRSLRS")) // 5

    fmt.Println(countCollisions1("RLRSLL")) // 5
    fmt.Println(countCollisions1("LLRR")) // 0
    fmt.Println(countCollisions1("LLLLLLLLL")) // 0
    fmt.Println(countCollisions1("RRRRRRRRR")) // 0
    fmt.Println(countCollisions1("SSSSSSSSS")) // 0
    fmt.Println(countCollisions1("LLLRRRSS")) // 3
    fmt.Println(countCollisions1("LRSLRSLRS")) // 5
}