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Java-based LeetCode algorithm problem solutions, regularly updated
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package g2201_2300.s2211_count_collisions_on_a_road;

// #Medium #String #Stack #2022_06_12_Time_113_ms_(45.96%)_Space_54.3_MB_(57.28%)

import java.util.ArrayDeque;
import java.util.Deque;

/**
 * 2211 - Count Collisions on a Road\.
 *
 * Medium
 *
 * 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 i^th 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'`.
**/
public class Solution {
    public int countCollisions(String directions) {
        if (directions == null || directions.length() == 1) {
            return 0;
        }
        Deque stack = new ArrayDeque<>();
        char[] direction = directions.toCharArray();
        char prevc = '0';
        int collision = 0;
        for (int i = 0; i < direction.length; i++) {
            if (direction[i] == 'R') {
                stack.push(direction[i]);
            } else {
                if ((direction[i] == 'S' && prevc == 'R')) {
                    if (!stack.isEmpty()) {
                        stack.pop();
                    }
                    collision += 1;
                    direction[i] = 'S';
                    while (!stack.isEmpty()) {
                        collision++;
                        stack.pop();
                    }
                }
                if ((direction[i] == 'L' && prevc == 'R')) {
                    stack.pop();
                    collision += 2;
                    direction[i] = 'S';
                    while (!stack.isEmpty()) {
                        collision++;
                        stack.pop();
                    }
                }
                if (direction[i] == 'L' && prevc == 'S') {
                    collision++;
                    direction[i] = 'S';
                }
            }
            prevc = direction[i];
        }
        return collision;
    }
}