Robot Bounded In Circle

LeetCode Q 1041 - Robot Bounded In Circle

On an infinite plane, a robot initially stands at (0, 0) and faces north. The robot can receive one of three instructions:

• "G": go straight 1 unit;
• "L": turn 90 degrees to the left;
• "R": turn 90 degress to the right.
  The robot performs the instructions given in order, and repeats them forever.

Return true if and only if there exists a circle in the plane such that the robot never leaves the circle.

Example 1: Input: "GGLLGG" ; Output: true
Explanation: The robot moves from (0,0) to (0,2), turns 180 degrees, and then returns to (0,0). When repeating these instructions, the robot remains in the circle of radius 2 centered at the origin.

Example 2: Input: "GG" ; Output: false
Explanation: The robot moves north indefinitely.

Example 3: Input: "GL" ; Output: true
Explanation: The robot moves from (0, 0) -> (0, 1) -> (-1, 1) -> (-1, 0) -> (0, 0) -> …

Note:

• 1 <= instructions.length <= 100
• instructions[i] is in {'G', 'L', 'R'}

Solution

Code:

public boolean isRobotBounded(String instructions) {
  int x = 0, y = 0, i = 0; 
  int[][] dirs = { {0, 1}, {-1, 0}, {0, -1}, {1, 0} };

  for (char ch: instructions.toCharArray()) {
    if (ch == 'L')
      i = (i + 1) % 4;
    else if (ch == 'R')
      i = (i + 3) % 4;
    else {
      x += dirs[i][0]; y += dirs[i][1];
    }
  }

  return x == 0 && y == 0 || i != 0;
}