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

class Solution {
    public boolean isRobotBounded(String instructions) {
        int i = 0, j = 0, dir = 1;
        for(char c : instructions.toCharArray()) {
            if(c == 'G') {
                if(dir == 1) j++;
                else if(dir == 2) i--;
                else if(dir == 3) j--;
                else i++;
            }
            else if(c == 'L') {
                dir = dir == 4 ? 1 : dir + 1;
            }
            else if(c == 'R') {
                dir = dir == 1 ? 4 : dir - 1;
            }
        }
        return i == 0 && j == 0 || dir > 1;
    }
}

https://leetcode.com/problems/robot-bounded-in-circle/discuss/731267/Easy-JAVA-Accepted-Solution

有circle条件：走一圈停在0，0，或者最后方向变了。

初始化（0， 0），dir 1，2，3，4代表N,W,S,E, 对应的方向是L时+1，R时-1

最后判断条件。