Two Pointers

function threeSum(nums: number[]): number[][] {
  const a = [...nums].sort((x, y) => x - y);
  const res: number[][] = [];
  for (let i = 0; i < a.length - 2; i++) {
    if (i > 0 && a[i] === a[i - 1]) continue; // skip duplicate anchor
    let lo = i + 1, hi = a.length - 1;
    while (lo < hi) {
      const sum = a[i] + a[lo] + a[hi];
      if (sum === 0) {
        res.push([a[i], a[lo], a[hi]]);
        lo++; hi--;
        while (lo < hi && a[lo] === a[lo - 1]) lo++; // skip dup values
        while (lo < hi && a[hi] === a[hi + 1]) hi--;
      } else if (sum < 0) lo++;
      else hi--;
    }
  }
  return res;
}

def three_sum(nums)
  a = nums.sort
  res = []
  (0..a.length - 3).each do |i|
    next if i.positive? && a[i] == a[i - 1] # skip duplicate anchor
    lo = i + 1
    hi = a.length - 1
    while lo < hi
      sum = a[i] + a[lo] + a[hi]
      if sum.zero?
        res << [a[i], a[lo], a[hi]]
        lo += 1
        hi -= 1
        lo += 1 while lo < hi && a[lo] == a[lo - 1] # skip dup values
        hi -= 1 while lo < hi && a[hi] == a[hi + 1]
      elsif sum.negative?
        lo += 1
      else
        hi -= 1
      end
    end
  end
  res
end