order_courses: Order college courses, given prereqs for each course.

Author: rayning0

order_courses_spec.rb

@@ -0,0 +1,47 @@
+# Given college courses, each with array of prereqs, return possible order of
+# courses to take, so you always take prereqs before each related course
+
+# Depth-first search algorithm with stack
+
+def order(hash)
+  ans, stack = [], []
+
+  hash.each do |id, pre|
+    if pre == []
+      ans << id.to_s if !ans.include?(id.to_s)
+      next
+    end
+
+    stack << id.to_s
+
+    while !stack.empty?
+      top = stack.last
+      if !ans.include?(top) && hash[top.to_sym] == []
+        ans << stack.pop
+      end
+
+      hash[top.to_sym].each do |prereq|
+        if !ans.include?(prereq) && !stack.include?(prereq) # not visited
+          stack << prereq
+          if !ans.include?(top) && prereq == []
+            ans << top
+          end
+        end
+      end
+
+      hash[top.to_sym] = [] # marks value of top as visited
+    end
+  end
+
+  ans
+end
+
+describe '#order' do
+  it 'returns articles in proper order, first showing prereqs' do
+    expect(order({'A': [], 'B': [], 'C': []})).to eq ['A', 'B', 'C']
+    expect(order({'A': ['B'], 'B': [], 'C': []})).to eq ['B', 'A', 'C']
+    expect(order({'A': ['B', 'C'], 'B': [], 'C': []})).to eq ['C', 'B', 'A']
+    expect(order({'A': [], 'B': ['C', 'D'], 'C': ['D'], 'D': []})).to eq ['A', 'D', 'C', 'B']
+    expect(order({'A': ['E'], 'B': ['C', 'D', 'E'], 'C': ['D', 'E'], 'D': [], 'E': []})).to eq ['E', 'A', 'D', 'C', 'B']
+  end
+end