二分検索/Binary Search

binary_search.c

int binary_search(int value, int values[], int left_index, int right_index) {
  int middle_index  = (right_index - left_index) / 2 + left_index;
  if (left_index > right_index) {
    return -1; 
  }
  else if (value < values[middle_index]) {
    return binary_search(value, values, left_index, middle_index - 1);
  }
  else if (value > values[middle_index]) {
    return binary_search(value, values, middle_index + 1, right_index);
  }
  else {
    return middle_index; 
  }
}

binary_search.rb

def binary_search(value, values, left_index = nil, right_index = nil)
  left_index  ||= 0
  right_index ||= values.size - 1
  middle_index  = (right_index - left_index) / 2 + left_index

  if left_index > right_index
    false
  elsif value < values[middle_index]
    binary_search(value, values, left_index, middle_index - 1)
  elsif value > values[middle_index]
    binary_search(value, values, middle_index + 1, right_index)
  else
    middle_index
  end
end

values  = (1 .. 999).to_a
p binary_search(0   , values) # false
p binary_search(1   , values) # 0
p binary_search(2   , values) # 1
p binary_search(500 , values) # 499
p binary_search(999 , values) # 998
p binary_search(1000, values) # false