export function permutations<T>(array: Array<T>): Array<Array<T>> { const ret: Array<Array<T>> = [];
if (array.length === 0) { return ret; }
function heapPermutations(k: number, array: Array<T>) { const c = new Array<number>(k).fill(0);
ret.push([...array]);
let i = 1;
while (i < k) { if (c[i] < i) { if (i % 2 === 0) { const swapBuffer = array[0]; array[0] = array[i]; array[i] = swapBuffer; } else { const swapBuffer = array[c[i]]; array[c[i]] = array[i]; array[i] = swapBuffer; }
ret.push([...array]);
c[i] += 1; i = 1; } else { c[i] = 0; i += 1; } } }
heapPermutations(array.length, [...array]);
return ret;}