combination_challenge.ts

import { expect } from "chai";

function buildCombinations(start: string, pieces: string[]): string[] {
  if (pieces.length < 1) {
    return [start];
  }

  const allCombinations: string[][] = pieces.map((newStart) => {
    let remaining = [...pieces];
    remaining.splice(remaining.indexOf(newStart), 1);
    const combs = buildCombinations(newStart, remaining);
    return combs.map((x) => start + x);
  });

  const results: string[] = [];
  allCombinations.forEach((row: string[]) => {
    row.forEach((x) => results.push(x));
  });
  return results;
}

function combinations(num: number): number[] {
  const tokens = String(num).split("");

  const results = buildCombinations("", tokens);

  return results.map((x) => Number(x));
}

describe("combinations", () => {
  it("returns a single item array for numbers less than 10", () => {
    expect(combinations(5)).to.deep.equal([5]);
  });

  it("returns all combinations of 2 digit numbers", () => {
    expect(combinations(42)).to.deep.equal([42, 24]);
  });

  it("returns all combinations of 3 digit numbers", () => {
    expect(combinations(421)).to.deep.equal([421, 412, 241, 214, 142, 124]);
  });

  it("example passes", () => {
    expect(combinations(123)).to.deep.equal([123, 132, 213, 231, 312, 321]);
  });

  it("returns all combinations of 3 digit numbers with duplicates", () => {
    expect(combinations(424)).to.deep.equal([424, 442, 244, 244, 424, 442]);
  });
});

results.txt

  combinations
    ✓ returns a single item array for numbers less than 10
    ✓ returns all combinations of 2 digit numbers
    ✓ returns all combinations of 3 digit numbers
    ✓ example passes
    ✓ returns all combinations of 3 digit numbers with duplicates