#### 原题说明

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: trueExplanation: 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: falseExplanation: The robot moves north indefinitely.`

Example 3:

`Input: “GL”Output: trueExplanation: 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’}`

#### 解题思路

1. 执行完一遍指令后回到原点
或者
2. 执行完一遍指令后发生转向（因为只有向左向右的转向，所以转向最终只可能是90，180，270）

`direction`的值 `0, 1, 2, 3` 分别表示 北，西，南，东

#### 归纳总结

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