# 147. Pick up stones

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There is a pile of `n`

(n > 0) stones.

Player A and Player B take turns to pick 1 or 2 stones from the pile. A starts first.

Who picks the last stone loses the game.

Now here is the question, **is there a winning strategy for A or B ?**
If so, returns the player name. If there is none, return null.

```
winningStonePicking(1)
// 'B'
winningStonePicking(2)
// 'A'
winningStonePicking(3)
// 'A'
winningStonePicking(4)
// 'B'
```