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1 """A red-black tree for Section 3.4, Exercise 4 in Allen Downey's _Think
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2 Complexity_ book.
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3
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4 http://greenteapress.com/complexity
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5
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6 Copyright (C) 2012 Brian G. Neal.
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7
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8 Permission is hereby granted, free of charge, to any person obtaining a copy of
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9 this software and associated documentation files (the "Software"), to deal in
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10 the Software without restriction, including without limitation the rights to
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11 use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
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12 the Software, and to permit persons to whom the Software is furnished to do so,
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13 subject to the following conditions:
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14
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15 The above copyright notice and this permission notice shall be included in all
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16 copies or substantial portions of the Software.
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17
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18 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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19 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
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20 FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
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21 COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
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22 IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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23 CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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24
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25 """
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26
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27 BLACK, RED = range(1)
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28
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29 class Node(object):
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30 """A node class for red-black trees.
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31
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32 A node has an optional parent, and optional left and right children. Each
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33 node also has a color, either red or black. A node has a key and an optional
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34 value. The key is used to order the red black tree by calling the "<"
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35 operator when comparing keys.
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36
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37 In a red-black tree, nil children are always considered black.
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38
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39 """
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40
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41 def __init__(self, key, color=RED, value=None):
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42 self.key = key
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43 self.value = value
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44 self.color = color
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45
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46 self.parent = None
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47 self.left = None
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48 self.right = None
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49
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50 @property
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51 def grandparent(self):
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52 """Return the grandparent of this node, or None if it does not exist."""
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53 return self.parent.parent if self.parent else None
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54
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55 @property
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56 def uncle(self):
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57 """Return this node's uncle if it exists, or None if not.
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58
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59 An uncle is a parent's sibling.
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60
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61 """
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62 g = self.grandparent
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63 if g:
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64 return g.left if g.right is self.parent else g.right
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65 return None
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