SE103 - Week 10, Session 1

Transcript

1. Week 10:
   Backtracking
   

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2. How was the HackerRank?
   

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3. What is Backtracking?
   

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4. What is Backtracking?
   Enter
   Exit
   

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5. What is Backtracking?
   Enter
   Exit
   What are we trying to solve?

   -> Ways to exit maze
   

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6. What is Backtracking?
   Enter
   Exit
   What are we trying to solve?

   -> Ways to exit maze
   1. Have we reached the end?

   -> If yes, we found a route!

   -> If no, try more paths (2)
   

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7. What is Backtracking?
   Enter
   Exit
   What are we trying to solve?

   -> Ways to exit maze
   1. Have we reached the end?

   -> If yes, we found a route!

   -> If no, try more paths (2)
   2. Go to next intersection / try
   next available routes
   

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8. What is Backtracking?
   Enter
   Exit
   What are we trying to solve?

   -> Ways to exit maze
   1. Have we reached the end?

   -> If yes, we found a route!

   -> If no, try more paths (2)
   2. Go to next intersection / try
   next available routes
   3. For each unvisited
   direction, try to solve again
   

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9. What is Backtracking?
   Enter
   Exit
   What are we trying to solve?

   -> Ways to exit maze
   1. Have we reached the end?

   -> If yes, we found a route!

   -> If no, try more paths (2)
   2. Go to next intersection / try
   next available routes
   3. For each unvisited
   direction, try to solve again
   4. If dead end, go back to last intersection
   

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10. - Backtracing: Uses constraints to make search
    more efficient. Usually looks for all answers that
    satisfy constraint.
    - Dynamic Programming: Looking for the most
    optimal solution
    Backtracking vs DP
    

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11. 1. Search - what are we searching / solving for?
    2. GetCandidates - get all the possible potential
    solutions (local solutions)
    3. IsSolution - is this candidate a solution?
    4. Backtrack - Undo any state that was caused with
    local guesses
    Common Backtracking Algorithm
    

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12. Given a 2D board and a word, find if the word exists in the grid. 

    

    The word can be constructed from letters of sequentially adjacent cell,
    where "adjacent" cells are those horizontally or vertically neighboring. The
    same letter cell may not be used more than once.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    “see” -> true
    “xyz” -> false
    Word Search
    

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13. Given a 2D board and a word, find if the word exists in the grid. 

    

    The word can be constructed from letters of sequentially adjacent cell,
    where "adjacent" cells are those horizontally or vertically neighboring. The
    same letter cell may not be used more than once.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    “see” -> true
    “xyz” -> false
    Word Search
    What are our constraints?
    

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14. Given a 2D board and a word, find if the word exists in the grid. 

    

    The word can be constructed from letters of sequentially adjacent cell,
    where "adjacent" cells are those horizontally or vertically neighboring.
    The same letter cell may not be used more than once.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    “see” -> true
    “xyz” -> false
    Word Search
    constraints
    

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15. Given a 2D board and a word, find if the word exists in the grid.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    Word Search
    isSolution(prefix, word)
    // prefix is our built up solution
    // return prefix == word
    getCandidates(position, visited, board, letter)
    
    // get unvisited adjacent positions that match
    letter
    
    backup(prefix, position, visited)
    
    // remove last letter from previous
    
    // remove position from visited
    
    search(board, word, prefix, position, visited)
    
    // return true if isSolution(prefix, word)
    
    // for each getCandidates
    
    // add letter(position) to prefix
    
    // add position to visited
    
    // search from next position
    
    // if not found backup
    

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16. Given a 2D board and a word, find if the word exists in the grid.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    Word Search
    isSolution(prefix, word)
    
    // prefix is our built up solution
    
    // return prefix == word
    
    getCandidates(position, visited, board, letter)
    // get unvisited adjacent positions that match
    letter
    backup(prefix, position, visited)
    
    // remove last letter from previous
    
    // remove position from visited
    
    search(board, word, prefix, position, visited)
    
    // return true if isSolution(prefix, word)
    
    // for each getCandidates
    
    // add letter(position) to prefix
    
    // add position to visited
    
    // search from next position
    
    // if not found backup
    

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17. Given a 2D board and a word, find if the word exists in the grid.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    Word Search
    isSolution(prefix, word)
    
    // prefix is our built up solution
    
    // return prefix == word
    
    getCandidates(position, visited, board, letter)
    
    // get unvisited adjacent positions that match
    letter
    
    backup(prefix, position, visited)
    // remove last letter from previous
    // remove position from visited
    search(board, word, prefix, position, visited)
    
    // return true if isSolution(prefix, word)
    
    // for each getCandidates
    
    // add letter(position) to prefix
    
    // add position to visited
    
    // search from next position
    
    // if not found backup
    

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18. Given a 2D board and a word, find if the word exists in the grid.
    board = [
    ['A','B','C','E'],
    ['S','F','C','S'],
    ['A','D','E','E']
    ]
    Word Search
    isSolution(prefix, word)
    
    // prefix is our built up solution
    
    // return prefix == word
    
    getCandidates(position, visited, board, letter)
    
    // get unvisited adjacent positions that match
    letter
    
    backup(prefix, position, visited)
    
    // remove last letter from previous
    
    // remove position from visited
    
    search(board, word, prefix, position, visited)
    // return true if isSolution(prefix, word)
    // for each getCandidates
    // add letter(position) to prefix
    // add position to visited
    // search from next position
    // if not found backup
    

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19. Debug Session
    Combination Sum
    

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20. Next Session
    - DP meets Backtracking!
    

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21. Breakout Rooms
    - Half of group does one problem, other half does the
    other problem
    
    - 30 minutes, everybody tries to do their own problem
    
    - 10 minutes - group 1 explains their problem
    
    - 10 minutes - group 2 explains their problem
    
    - Last 10 minutes - try implementing the problem we
    just debugged together!
    

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22. Hiring Behind the Scenes
    What happens after you finish that onsite interview??
    

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23. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    

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24. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    - Everybody gets together to talk as a group
    
    

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25. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    - Everybody gets together to talk as a group
    
    - Was the person’s coding skills up to par? (did they understand
    time / space complexity, do they know when to use different data
    structures, etc)

    

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26. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    - Everybody gets together to talk as a group
    
    - Was the person’s coding skills up to par? (did they understand
    time / space complexity, do they know when to use different data
    structures, etc)

    - Was the person able to communicate well?

    

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27. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    - Everybody gets together to talk as a group
    
    - Was the person’s coding skills up to par? (did they understand
    time / space complexity, do they know when to use different data
    structures, etc)

    - Was the person able to communicate well?

    - Was the person respectful of everybody?
    
    

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28. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    - Everybody gets together to talk as a group
    
    - Was the person’s coding skills up to par? (did they understand
    time / space complexity, do they know when to use different data
    structures, etc)

    - Was the person able to communicate well?

    - Was the person respectful of everybody?
    
    - Is this someone we would enjoy working with?

    

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29. Hiring Behind the Scenes
    Startup World:

    - Everybody individually enters their feedback right after they talk to
    you (hire / no hire / not sure)
    
    - Everybody gets together to talk as a group
    
    - Was the person’s coding skills up to par? (did they understand
    time / space complexity, do they know when to use different data
    structures, etc)

    - Was the person able to communicate well?

    - Was the person respectful of everybody?
    
    - Is this someone we would enjoy working with?

    - Does the person’s background match what we need in the
    team?
    

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30. Hiring Behind the Scenes
    Big Company World
    
    

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31. Hiring Behind the Scenes
    Big Company World
    
    - Interviewer enters their feedback immediately after

    

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32. Hiring Behind the Scenes
    Big Company World
    
    - Interviewer enters their feedback immediately after

    1. Coding
    
    2. Design
    
    3. Communication
    
    4. Speed
    

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33. Hiring Behind the Scenes
    Big Company World

    

    - Coding skills

    - is your code easy to understand?

    - did you handle edge cases?

    - do you know when to use data structures and built in methods?

    - were you able to solve your own bugs?
    
    - Design
    
    - Communication
    
    - Speed

    

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34. Hiring Behind the Scenes
    Big Company World

    - Coding

    - Design

    - were you able to gather enough requirements to solve the problem?
    
    - were you able to discuss trade-offs with different approaches?

    - were you able to verbally explain a solution before jumping into
    implementation?

    - did you use the appropriate data structures / algorithms?
    
    - Communication
    
    - Speed

    

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35. Hiring Behind the Scenes
    Big Company World
    
    

    - Coding
    
    - Design
    
    - Communication

    - were you able to understand the problem and state possible
    confusions?

    - did what you say match what you coded?
    
    - Speed

    

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36. Hiring Behind the Scenes
    Big Company World
    
    

    - Coding
    
    - Design
    
    - Communication
    
    - Speed

    - were you able to finish the question within the expected time?
    (often times a question has followups)

    - did you need hints to guide you to the solution?
    

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