2751. Robot Collisions LeetCode Solution

Last updated: March 5, 2025

In this guide, you will get 2751. Robot Collisions LeetCode Solution with the best time and space complexity. The solution to Robot Collisions problem is provided in various programming languages like C++, Java, and Python. This will be helpful for you if you are preparing for placements, hackathons, interviews, or practice purposes. The solutions provided here are very easy to follow and include detailed explanations.

Problem Statement
Complexity Analysis
Robot Collisions solution in C++
Robot Collisions solution in Java
Robot Collisions solution in Python
Additional Resources

2751. Robot Collisions LeetCode Solution image

Problem Statement of Robot Collisions

There are n 1-indexed robots, each having a position on a line, health, and movement direction.
You are given 0-indexed integer arrays positions, healths, and a string directions (directions[i] is either ‘L’ for left or ‘R’ for right). All integers in positions are unique.
All robots start moving on the line simultaneously at the same speed in their given directions. If two robots ever share the same position while moving, they will collide.
If two robots collide, the robot with lower health is removed from the line, and the health of the other robot decreases by one. The surviving robot continues in the same direction it was going. If both robots have the same health, they are both removed from the line.
Your task is to determine the health of the robots that survive the collisions, in the same order that the robots were given, i.e. final health of robot 1 (if survived), final health of robot 2 (if survived), and so on. If there are no survivors, return an empty array.
Return an array containing the health of the remaining robots (in the order they were given in the input), after no further collisions can occur.
Note: The positions may be unsorted.

Example 1:

Input: positions = [5,4,3,2,1], healths = [2,17,9,15,10], directions = “RRRRR”
Output: [2,17,9,15,10]
Explanation: No collision occurs in this example, since all robots are moving in the same direction. So, the health of the robots in order from the first robot is returned, [2, 17, 9, 15, 10].

Example 2:

Input: positions = [3,5,2,6], healths = [10,10,15,12], directions = “RLRL”
Output: [14]
Explanation: There are 2 collisions in this example. Firstly, robot 1 and robot 2 will collide, and since both have the same health, they will be removed from the line. Next, robot 3 and robot 4 will collide and since robot 4’s health is smaller, it gets removed, and robot 3’s health becomes 15 – 1 = 14. Only robot 3 remains, so we return [14].

Example 3:

Input: positions = [1,2,5,6], healths = [10,10,11,11], directions = “RLRL”
Output: []
Explanation: Robot 1 and robot 2 will collide and since both have the same health, they are both removed. Robot 3 and 4 will collide and since both have the same health, they are both removed. So, we return an empty array, [].

Constraints:

1 <= positions.length == healths.length == directions.length == n <= 105
1 <= positions[i], healths[i] <= 109
directions[i] == 'L' or directions[i] == 'R'
All values in positions are distinct

Complexity Analysis

Time Complexity: O(n)
Space Complexity: O(n)

2751. Robot Collisions LeetCode Solution in C++

struct Robot {
  int index;
  int position;
  int health;
  char direction;
};

class Solution {
 public:
  vector<int> survivedRobotsHealths(vector<int>& positions,
                                    vector<int>& healths, string directions) {
    vector<int> ans;
    vector<Robot> robots;
    vector<Robot> stack;  // the runnnig robots

    for (int i = 0; i < positions.size(); ++i)
      robots.push_back(Robot{i, positions[i], healths[i], directions[i]});

    ranges::sort(robots, ranges::less{},
                 [](const Robot& robot) { return robot.position; });

    for (Robot& robot : robots) {
      if (robot.direction == 'R') {
        stack.push_back(robot);
        continue;
      }
      // Collide with robots going right if any.
      while (!stack.empty() && stack.back().direction == 'R' &&
             robot.health > 0) {
        if (stack.back().health == robot.health) {
          stack.pop_back();
          robot.health = 0;
        } else if (stack.back().health < robot.health) {
          stack.pop_back();
          robot.health -= 1;
        } else {  // stack.back().health > robot.health
          stack.back().health -= 1;
          robot.health = 0;
        }
      }
      if (robot.health > 0)
        stack.push_back(robot);
    }

    ranges::sort(stack, ranges::less{},
                 [](const Robot& robot) { return robot.index; });

    for (const Robot& robot : stack)
      ans.push_back(robot.health);

    return ans;
  }
};
/* code provided by PROGIEZ */

2751. Robot Collisions LeetCode Solution in Java

class Robot {
  public int index;
  public int position;
  public int health;
  public char direction;
  public Robot(int index, int position, int health, char direction) {
    this.index = index;
    this.position = position;
    this.health = health;
    this.direction = direction;
  }
}

class Solution {
  public List<Integer> survivedRobotsHealths(int[] positions, int[] healths, String directions) {
    List<Integer> ans = new ArrayList<>();
    Robot[] robots = new Robot[positions.length];
    List<Robot> stack = new ArrayList<>(); // running robots

    for (int i = 0; i < positions.length; ++i)
      robots[i] = new Robot(i, positions[i], healths[i], directions.charAt(i));

    Arrays.sort(robots, (a, b) -> Integer.compare(a.position, b.position));

    for (Robot robot : robots) {
      if (robot.direction == 'R') {
        stack.add(robot);
        continue;
      }
      // Collide with robots going right if any.
      while (!stack.isEmpty() && stack.get(stack.size() - 1).direction == 'R' && robot.health > 0) {
        if (stack.get(stack.size() - 1).health == robot.health) {
          stack.remove(stack.size() - 1);
          robot.health = 0;
        } else if (stack.get(stack.size() - 1).health < robot.health) {
          stack.remove(stack.size() - 1);
          robot.health -= 1;
        } else { // stack[-1].health > robot.health
          stack.get(stack.size() - 1).health -= 1;
          robot.health = 0;
        }
      }
      if (robot.health > 0)
        stack.add(robot);
    }

    stack.sort((a, b) -> Integer.compare(a.index, b.index));

    for (Robot robot : stack)
      ans.add(robot.health);

    return ans;
  }
}
// code provided by PROGIEZ

2751. Robot Collisions LeetCode Solution in Python

from dataclasses import dataclass


@dataclass
class Robot:
  index: int
  position: int
  health: int
  direction: str


class Solution:
  def survivedRobotsHealths(
      self,
      positions: list[int],
      healths: list[int],
      directions: str,
  ) -> list[int]:
    robots = sorted([Robot(index, position, health, direction)
                     for index, (position, health, direction) in
                     enumerate(zip(positions, healths, directions))],
                    key=lambda x: x.position)
    stack: list[Robot] = []  # running robots

    for robot in robots:
      if robot.direction == 'R':
        stack.append(robot)
        continue
      # Collide with robots going right if any.
      while stack and stack[-1].direction == 'R' and robot.health > 0:
        if stack[-1].health == robot.health:
          stack.pop()
          robot.health = 0
        elif stack[-1].health < robot.health:
          stack.pop()
          robot.health -= 1
        else:  # stack[-1].health > robot.health
          stack[-1].health -= 1
          robot.health = 0
      if robot.health > 0:
        stack.append(robot)

    stack.sort(key=lambda robot: robot.index)
    return [robot.health for robot in stack]
 # code by PROGIEZ

Additional Resources

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