Boggle
Intro to recursive backtracking

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Intro to Boggle
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Intro to recursive backtracking
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Practice (with an APT)
Today

How to play

How to play

How to find words?
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You have a dictionary
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You want to find all words on board
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How?
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Write out step-bystep algorithm

Brute force
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Find everything!
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N
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N-Z
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N-Z-I
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N-Z-V
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N-Z-E
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N-Z-A
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N-E
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N-A

Is there a better way?
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Find everything!
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N
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N-Z
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N-Z-I
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N-Z-V
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N-Z-E
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N-Z-A
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N-E
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N-A

Is there a better way?
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Find everything!
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N
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N-Z
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N-Z-I
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N-Z-V
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N-Z-E
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N-Z-A
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N-E
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N-A

Prefixes
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Look at word prefixes
• If prefix doesn’t make a word, STOP
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We should store our words in a data structure to help us

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Tree-like structure
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Node
• the character to reach node
• if the node forms a word
• denoted in red
• map of children
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<Character, Node>
• parent node
Tries

Let’s trie it!
• “Peter piper picked a peck of pickled peppers”

Recursion void solve (ProblemClass instance){ base cases recursion reassemble problem
}

Bracktracking
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Classic problem – N Queens
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On an NxN board, how do you place N queens such that no queens can attack each other
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Backtracking
• place a queen on board
• if ok, keep going
• else, remove queen
4/11/2020 13

Recursive backtracking void solve (ProblemClass instance){ base cases save current state recursion if current state DIDN’T work remove current state reassemble problem
}

4/11/2020
Grid Game
15