Introduction

In this blog post, we will explore how to solve the LeetCode problem "2211" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

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 <= 105 directions[i] is either 'L', 'R', or 'S'.

Explanation

Here's the breakdown of the problem and the Python solution:

• High-Level Approach:

  • Iterate through the directions string, focusing on consecutive pairs of cars.
  • Identify collision scenarios based on the directions of adjacent cars: 'RL' (collision of 2), 'RS' or 'SL' (collision of 1).
  • Keep track of the total collisions and the state of previously collided cars (effectively stationary 'S').

• Complexity:

  • Runtime Complexity: O(n) - single pass through the string.
  • Storage Complexity: O(1) - constant extra space.

• Python Code:

Code

    def ountollisions(diretions: str) -> int:    collisions = 0
    n = len(directions)

    for i in range(n - 1):
        if directions[i] == 'R' and directions[i+1] == 'L':
            collisions += 2
            directions = directions[:i] + 'S' + 'S' + directions[i+2:]
        elif directions[i] == 'R' and directions[i+1] == 'S':
            collisions += 1
            directions = directions[:i] + 'S' + 'S' + directions[i+2:]
        elif directions[i] == 'S' and directions[i+1] == 'L':
            collisions += 1
            directions = directions[:i+1] + 'S' + directions[i+2:]

    return collisions