Implement Fisher-Yates Shuffle (in-place, unbiased)

Question

Accepted Answer

**Approach:** Iterate from end, swap `i` with random `j` in `[0,i]`. Copy first to avoid mutating input.

```javascript
function shuffle(array) {
  const a = [...array];
  for (let i = a.length - 1; i > 0; i--) {
    const j = Math.floor(Math.random() * (i + 1));
    [a[i], a[j]] = [a[j], a[i]];
  }
  return a;
}

// Demo
console.log(shuffle([1,2,3,4,5])); // e.g. [3,1,5,2,4] (random)
console.log(shuffle([])); // []
console.log(shuffle([1])); // [1]
function testUniform() {
  const counts = {};
  for (let i=0;i<6000;i++) {
    const s = shuffle([1,2,3]).join(',');
    counts[s] = (counts[s]||0)+1;
  }
  console.log(counts); // each of 6 perms ~1000
}
testUniform();
function shuffleSeeded(arr, rng = Math.random) {
  const a=[...arr];
  for(let i=a.length-1;i>0;i--){ const j=Math.floor(rng()*(i+1)); [a[i],a[j]]=[a[j],a[i]]; }
  return a;
}
```
Never use `arr.sort(()=>Math.random()-0.5)` — biased and O(n log n).

Implement Fisher-Yates Shuffle (in-place, unbiased)

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