Backtracking

function combinationSum(candidates: number[], target: number): number[][] {
  const res: number[][] = [];
  const path: number[] = [];
  const backtrack = (start: number, remaining: number): void => {
    if (remaining === 0) { res.push([...path]); return; }
    if (remaining < 0) return;
    for (let i = start; i < candidates.length; i++) {
      path.push(candidates[i]);
      backtrack(i, remaining - candidates[i]); // i, not i+1: reuse allowed
      path.pop();
    }
  };
  backtrack(0, target);
  return res;
}

def combination_sum(candidates, target)
  res = []
  path = []
  backtrack = lambda do |start, remaining|
    if remaining.zero?
      res << path.dup
      return
    end
    return if remaining.negative?

    (start...candidates.length).each do |i|
      path.push(candidates[i])
      backtrack.call(i, remaining - candidates[i]) # i, not i+1: reuse allowed
      path.pop
    end
  end
  backtrack.call(0, target)
  res
end