solve: Find Median from Data Stream

Author: 0x01f7

README.md

@@ -4,7 +4,7 @@
 |  #  | Title                                                               | Solution      | Difficulty  |
 |-----|---------------------------------------------------------------------|---------------|-------------|
 | 296 | [Best Meeting Point][p296]                                          |    :lock:     | Medium      |
-| 295 | [Find Median from Data Stream][p295]                                |               | Hard        |
+| 295 | [Find Median from Data Stream][p295]                                | [Ruby][s295]  | Hard        |
 | 294 | [Flip Game II][p294]                                                |    :lock:     | Medium      |
 | 293 | [Flip Game][p293]                                                   |    :lock:     | Easy        |
 | 292 | [Nim Game][p292]                                                    | [Ruby][s292]  | Easy        |
@@ -566,6 +566,7 @@
 [p2]:https://leetcode.com/problems/add-two-numbers/
 [p1]:https://leetcode.com/problems/two-sum/
 
+[s295]:./algorithms/find_median_from_data_stream.rb
 [s292]:./algorithms/nim_game.rb
 [s290]:./algorithms/word_pattern.rb
 [s283]:./algorithms/move_zeroes.rb

algorithms/find_median_from_data_stream.rb

@@ -0,0 +1,111 @@
+# https://leetcode.com/problems/find-median-from-data-stream/
+#
+# Median is the middle value in an ordered integer list. If the size of
+# the list is even, there is no middle value. So the median is the mean
+# of the two middle value.
+#
+# Examples:
+#
+#     [2,3,4] , the median is 3
+#     [2,3], the median is (2 + 3) / 2 = 2.5
+#
+# Design a data structure that supports the following two operations:
+#
+#     + void addNum(int num) - Add a integer number from the data stream
+#       to the data structure.
+#     + double findMedian() - Return the median of all elements so far.
+#
+# For example:
+#
+#     add(1)
+#     add(2)
+#     findMedian() -> 1.5
+#     add(3)
+#     findMedian() -> 2
+
+
+class PriorityQueue
+  def initialize(cmp)
+    @queue, @cmp = [], cmp
+  end
+
+  def <<(value)
+    @queue << value; _swim_(@queue.size - 1); self
+  end
+
+  def shift
+    @queue[0], @queue[-1] = @queue[-1], @queue[0]
+    @queue.pop.tap { _sink_(0) }
+  end
+
+  def peek
+    @queue[0]
+  end
+
+  def size
+    @queue.size
+  end
+
+  def empty?
+    @queue.empty?
+  end
+
+  private def _swim_(k)
+    while k > 0 && @cmp.call(@queue[k], @queue[(k - 1) / 2])
+      @queue[k], @queue[(k - 1) / 2] = @queue[(k - 1) / 2], @queue[k]
+      k = (k - 1) / 2
+    end
+  end
+
+  private def _sink_(k)
+    while k * 2 + 1 < @queue.size
+      kk  = k * 2 + 1
+      kk += 1 if @queue[kk + 1] && @cmp.call(@queue[kk + 1], @queue[kk])
+      break if @cmp.call(@queue[k], @queue[kk])
+
+      @queue[k], @queue[kk] = @queue[kk], @queue[k]
+      k = kk
+    end
+  end
+end
+
+
+class MedianFinder
+  # Initialize your data structure here.
+  def initialize
+    @maxpq = PriorityQueue.new(->(x1, x2) { x1 > x2 })
+    @minpq = PriorityQueue.new(->(x1, x2) { x1 < x2 })
+  end
+
+  # @param {integer} num
+  # @return {void}
+  # Adds a num into the data structure.
+  def add_num(num)
+    if @maxpq.empty?
+      @maxpq << num; return
+    end
+
+    if @maxpq.size == @minpq.size
+      @maxpq << num
+    else
+      @minpq << num
+    end
+
+    if @maxpq.peek > @minpq.peek
+      @maxpq << @minpq.shift
+      @minpq << @maxpq.shift
+    end; nil
+  end
+
+  # @return {double}
+  # Returns median of current data stream
+  def find_median
+    @maxpq.size == @minpq.size ? (@maxpq.peek + @minpq.peek).fdiv(2) : @maxpq.peek
+  end
+end
+
+
+# Your MedianFinder object will be instantiated and called as such:
+# mf = MedianFinder.new
+# mf.add_num(1)
+# mf.find_median()