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 degrees 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: instructions = "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: instructions = "GG"
Output: false
Explanation: The robot moves north indefinitely.

Example 3:

Input: instructions = "GL"
Output: true
Explanation: The robot moves from (0, 0) -> (0, 1) -> (-1, 1) -> (-1, 0) -> (0, 0) -> ...

Constraints:

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

这是一道类似脑筋急转弯的题目——你想到了那个点就可以解出来，想不到就无从下手。所以我不喜欢这一题。我记录它的原因是它涉及到对转向的一般性解法，除此之外我不打算多费唇舌。如果有人好奇可以去B战搜索，有人做了极好的视频。

/**
 * @param {string} instructions
 * @return {boolean}
 */
var isRobotBounded = function(instructions) {
    const location = [0, 0]
    let d = 0
    // 注意这个二维数组
    const directionM = [
        [0,1],
        [-1,0],
        [0,-1],
        [1,0]
    ]
    
    function parse(command, location) {
        if (command === 'G') {
            let add = directionM[d]
            location[0] += add[0]
            location[1] += add[1]
        }
        // 以及我们是怎么让机器人转向的。
        if (command === 'L') {
            d = (d + 1) % 4
        }
        if (command === 'R')
            d = (d - 1 + 4) % 4
    }
    
    instructions.split('').forEach(c => {
        parse(c, location)
    })
    
    if (
        (location [0] === 0 && location[1] === 0) || d !== 0
    )  {
        return true    
    }
    
    return false
};