Sync LeetCode submission Runtime - 4 ms (96.59%), Memory - 18.1 MB (98.29%)

Author: jimit105

2317-count-collisions-on-a-road/README.md

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+<p>There are <code>n</code> cars on an infinitely long road. The cars are numbered from <code>0</code> to <code>n - 1</code> from left to right and each car is present at a <strong>unique</strong> point.</p>
+
+<p>You are given a <strong>0-indexed</strong> string <code>directions</code> of length <code>n</code>. <code>directions[i]</code> can be either <code>&#39;L&#39;</code>, <code>&#39;R&#39;</code>, or <code>&#39;S&#39;</code> denoting whether the <code>i<sup>th</sup></code> car is moving towards the <strong>left</strong>, towards the <strong>right</strong>, or <strong>staying</strong> at its current point respectively. Each moving car has the <strong>same speed</strong>.</p>
+
+<p>The number of collisions can be calculated as follows:</p>
+
+<ul>
+	<li>When two cars moving in <strong>opposite</strong> directions collide with each other, the number of collisions increases by <code>2</code>.</li>
+	<li>When a moving car collides with a stationary car, the number of collisions increases by <code>1</code>.</li>
+</ul>
+
+<p>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.</p>
+
+<p>Return <em>the <strong>total number of collisions</strong> that will happen on the road</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> directions = &quot;RLRSLL&quot;
+<strong>Output:</strong> 5
+<strong>Explanation:</strong>
+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. 
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> directions = &quot;LLRR&quot;
+<strong>Output:</strong> 0
+<strong>Explanation:</strong>
+No cars will collide with each other. Thus, the total number of collisions that will happen on the road is 0.</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= directions.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>directions[i]</code> is either <code>&#39;L&#39;</code>, <code>&#39;R&#39;</code>, or <code>&#39;S&#39;</code>.</li>
+</ul>

2317-count-collisions-on-a-road/solution.py

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+# Approach 2: Counting
+
+# Time: O(n)
+# Space: O(1)
+
+class Solution:
+    def countCollisions(self, directions: str) -> int:
+        dirs = directions.lstrip('L').rstrip('R')
+        return len(dirs) - dirs.count('S')