1
var Solution = function (m, n) {
2
  this.length = m * n;
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  this.range = new Array(this.length);
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  this.n = n;
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  this.pointer = 0;
6
};
7

8
Solution.prototype.flip = function () {
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  let limit = this.length - this.pointer - 1;
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  let index = ~~(Math.random() * limit) + this.pointer;
11

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  if (this.range[index] === undefined) this.range[index] = index;
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  if (this.range[this.pointer] === undefined)
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    this.range[this.pointer] = this.pointer;
15

16
  let tmp = this.range[index];
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  this.range[index] = this.range[this.pointer];
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  this.range[this.pointer] = tmp;
19

20
  let a = ~~(this.range[this.pointer] / this.n);
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  let b = this.range[this.pointer] % this.n;
22

23
  this.pointer++;
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  return [a, b];
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};
26

27
Solution.prototype.reset = function () {
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  this.pointer = 0;
29
};