four_sum.rb

# Originally Accepted at least on 20/aug/2015, but TLE on 31/may/2022
# TLE solved by ignoring repetition probing on 31/may/2022
def four_sum(nums, target)
  nums.sort!
  # create hash with default value:[]
  two_sum, result = Hash.new { |h, k| h[k] = [] }, []
  nums.size.times do |i|
    (i + 1...nums.size).each do |j|
      two_sum[nums[i] + nums[j]] += [[i, j]]
    end
  end
  nums.size.times do |i|
    # important speedup to remove repeated calculation
    next if nums[i] == nums[i - 1] && i > 0
    (i + 1...nums.size - 2).each do |j|
      #important condition: j > i + 1 , j = i + 1 need to be reached once at least
      next if nums[j] == nums[j - 1] && j > i + 1
      expected = target - nums[i] - nums[j]
      two_sum[expected].each do |a, b|
        # ensure the order
        result << [nums[i], nums[j], nums[a], nums[b]] if a > j
      end
    end
  end
  result.uniq
end

# DFS solution but TLE on 31/may/2022
def four_sum_dfs(nums, target)
  nums.sort!
  result = []
  dfs_search(nums, 0, 4, target, [], result)
  result.uniq
end

def dfs_search(nums, i, remain_steps, target, cur_list, result)
  return cur_list.sum == target ? result << cur_list : nil if remain_steps == 0
  (i...nums.size).each do |j|
    next if nums[j] == nums[j - 1] && j > i
    dfs_search(nums, j + 1, remain_steps - 1, target, cur_list + [nums[j]], result)
  end
end

def four_sum_with_2_pointers(nums, target)
  nums.sort!
  result = []
  nums.size.times do |i|
    next if i > 0 && nums[i] == nums[i - 1]
    (i + 1...nums.size).each do |j|
      next if j > i + 1 && nums[j] == nums[j - 1]
      left, right = j + 1, nums.size - 1
      while left < right
        case nums[i] + nums[j] + nums[left] + nums[right] <=> target
        when 0 # <=> outputs 0 when equal
          result << [nums[i], nums[j], nums[left], nums[right]]
          left += 1 while nums[left] == nums[left + 1]
          right -= 1 while nums[right] == nums[right - 1]
          left, right = left + 1, right - 1
        when -1 # <=> smaller
          left += 1 while nums[left] == nums[left + 1]
          left += 1
        when 1 # larger
          right -= 1 while nums[right] == nums[right - 1]
          right -= 1
        end
      end
    end
  end
  result
end