Recursion Tree Method in Algorithm Analysis
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Questions and Answers

What is the base case for the recurrence relation T(n) = 3T(n/4) + n^2?

  • T(n)
  • T(n/4)
  • T(0)
  • T(1) (correct)
  • The recursion tree method is a technique used to solve recurrence relations.

    True (A)

    What is the total number of nodes at level 'i' in the recursion tree?

    3^i

    What is the recurrence relation for the given problem?

    <p>T(n) = 3T(n/4) + n^2 (B)</p> Signup and view all the answers

    The impact of each node at level 'i' on the recurrence relation is ______.

    <p>n^2 / 16^i</p> Signup and view all the answers

    Match the following concepts to their descriptions:

    <p>Recurrence relation = An equation that defines a sequence recursively, meaning each term is expressed in terms of previous terms. Base case = The condition that terminates the recursion. Recursion tree method = A visual technique used to solve recurrence relations by representing the recursive calls as a tree. Node = A representation of a recursive call in the recursion tree.</p> Signup and view all the answers

    The recursion tree method helps visualize the recursive calls made in a problem.

    <p>True (A)</p> Signup and view all the answers

    What is the base case for the recurrence relation?

    <p>T(1)</p> Signup and view all the answers

    The impact of each node in the recursion tree at layer i is ______

    <p>3^i * n^2 / 16^i</p> Signup and view all the answers

    Match the following terms with their corresponding values in the recursion tree method:

    <p>Node Count (Layer 0) = 1 Impact (Layer 1) = 3n^2/16 Node Count (Layer 1) = 3 Impact (Layer 2) = 9n^2/256</p> Signup and view all the answers

    What is the initial step we take to prove that a time complexity function T(n) is in O(n) after introducing the claim T(n) ∈ O(n)?

    <p>Use induction on n to prove T(n) ≤ cn - d for constants c, d &gt; 0. (C)</p> Signup and view all the answers

    The initial induction hypothesis states that T(k) ≤ ck - d for all k < n, where c and d are constants.

    <p>True (A)</p> Signup and view all the answers

    What is the main tool used in the provided proof to demonstrate the claim T(n) ∈ O(n)?

    <p>Mathematical Induction</p> Signup and view all the answers

    In the provided proof, the equation T(n) = T(⌊n/2⌋) + T(⌈n/2⌉) + 1 is used to expand the time complexity function based on the assumption that T(n) is in ______ of the size of the input n.

    <p>linear</p> Signup and view all the answers

    Match the following terms with their corresponding definitions in the provided context.

    <p>⌊n/2⌋ = The largest integer less than or equal to n/2 ⌈n/2⌉ = The smallest integer greater than or equal to n/2 c = A constant that determines the upper bound of the function d = A constant that represents the additional factor in the upper bound T(n) = The time complexity function defining the time taken for an algorithm to run on an input of size n</p> Signup and view all the answers

    The recursion tree method is used to analyze the time complexity of ______ functions.

    <p>recursive</p> Signup and view all the answers

    In the recursion tree for T(n) = 3T(n/4) + n^2, what is the value at each node at the first level?

    <p>n^2 (B)</p> Signup and view all the answers

    What is the base case for the recursive function T(n) = 3T(n/4) + n^2?

    <p>T(1) = 1</p> Signup and view all the answers

    The recursion tree method can be used to analyze the time complexity of all recursive functions.

    <p>True (A)</p> Signup and view all the answers

    In the recursion tree for T(n) = 3T(n/4) + n^2, how many nodes are there at the second level?

    <p>9 (D)</p> Signup and view all the answers

    The recursion tree method helps to visualize how the time complexity of a recursive function ______ as the problem size decreases.

    <p>evolves</p> Signup and view all the answers

    What is the main advantage of using the recursion tree method in analyzing time complexity?

    <p>It provides a visual representation of the recursive calls and their associated costs.</p> Signup and view all the answers

    Match the following terms with their corresponding description:

    <p>T(n) = Time complexity function n^2 = Cost of a single recursive call 3T(n/4) = Recursive calls T(1) = 1 = Base case</p> Signup and view all the answers

    The recurrence relation presented in the content is T(n) = 3T(n/4) + ______

    <p>n^2</p> Signup and view all the answers

    What is the time complexity of the algorithm represented by the recurrence relation?

    <p>O(n^2) (D)</p> Signup and view all the answers

    The recursion tree method is a way to visualize and solve recurrence relations.

    <p>True (A)</p> Signup and view all the answers

    What does the 'n^2' term in the recurrence relation represent?

    <p>The work done at each level of the recursion tree</p> Signup and view all the answers

    Match the following terms with their corresponding elements in the recursion tree method:

    <p>Node Impact = The amount of work done at a particular node Layer Count = The number of nodes at a specific level of the tree Layer = A specific level in the recursion tree</p> Signup and view all the answers

    What is the recurrence relation for T(n)?

    <p>T(n) = 3T(n/4) + n^2 (D)</p> Signup and view all the answers

    The impact of each node at layer 0 in the recursion tree is n^2.

    <p>True (A)</p> Signup and view all the answers

    What is the value of T(1)?

    <p>1</p> Signup and view all the answers

    In the recursion tree method, the impact of each node is represented as _____ at layer 1.

    <p>16n</p> Signup and view all the answers

    How many nodes are there at layer 2 of the recursion tree?

    <p>16 (D)</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>T(n) = Total time complexity n^2 = Node impact per layer log4 n = Logarithmic term in the function i = Layer counter in the recursion tree</p> Signup and view all the answers

    The total number of nodes in the recursion tree increases exponentially with each layer.

    <p>True (A)</p> Signup and view all the answers

    What expression represents the total work done at a layer i?

    <p>n^(2) * 3^i</p> Signup and view all the answers

    Flashcards

    Recursion tree method

    A visual method for analyzing the time complexity of recursive algorithms.

    T(n) = 3T(n/4) + n²

    A specific recurrence relation representing time complexity in algorithms.

    Node Impact Count

    The contribution of each node's calculation to the overall complexity.

    Layer in recursion tree

    Levels in the recursion tree indicating stages of recursive calls.

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    Impact at layer 0

    The initial layer where the primary input size affects the complexity.

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    Base Case

    The condition that stops the recursion; here T(1) = 1.

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    Depth of Recursion Tree

    The levels in the recursion tree until the base case is reached.

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    Subproblems in Recursion

    Smaller instances of the original problem at each level of the tree.

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    Work Done at Each Level

    The amount of work performed at each level of recursion.

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    Total Work in Recursion

    The sum of work done at all levels of the recursion tree.

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    Big O notation

    A mathematical notation that describes the limiting behavior of a function when the argument tends towards infinity.

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    Complexity Analysis

    Determining the time and space complexity of the recursive function.

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    Recurrence Relation

    An equation that defines a sequence based on previous terms.

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    T(n)

    The function representing the time complexity for input size n.

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    O(n)

    Big O notation indicating linear time complexity.

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    Inductive Proof

    A method of proving statements true using induction principle.

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    Inductive Hypothesis (IH)

    Assumption made for the inductive step that the statement holds for all k<n.

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    Constants c and d

    Specific values used to make time complexity claims tighter.

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    Tight Bound

    A more specific claim about the upper bound of T(n).

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    Total impact at layer

    The sum of impacts of all nodes at a particular layer in the recursion tree.

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    T(n) Recursion

    T(n) = 3T(n/4) + n^2 describes a recurrence relation.

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    Base case T(1)

    T(1) = 1 is the base case for the recursion.

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    Layer count

    Each layer of the recursion tree has n^2 impact.

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    Node Impact

    Node impact increases exponentially in the recursion tree.

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    Depth of Recursion

    The depth determines how many times the problem is divided.

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    Total Cost T(n)

    Total cost is the sum of all node impacts across layers.

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    Logarithmic Cost

    log4 n reflects a logarithmic impact in the layer contributions.

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    Polynomial Growth

    n^2 describes polynomial growth affecting the cost calculation.

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    Study Notes

    Algorithms and Data Structures

    • Lecture topic: Recurrence Relations
    • Lecture date: Winter Term 2024/25, 5th lecture

    Solving Recurrence Relations

    • Question: Does T(n) = 2 · T(n/2) + 4n (with T(1) = 0) imply T(n) ∈ O(nlogn)?
    • Claim: There exists a c > 0 such that T(n) ≤ cn log₂ n.
    • Proof: By induction over n.
      • Base case: T(1) ≤ 0
      • Inductive hypothesis: T(k) ≤ ck log₂ k holds for all k < n.
      • Using the recurrence relation and the inductive hypothesis: T(n) = 2T(n/2) + 4n ≤ 2c (n/2) log₂(n/2) + 4n = 2 cn log₂ n - 2cn log₂ 2 + 4n = 2 cn log₂ n - 2cn + 4n
      • For T(n) ≤ cn log₂ n, we need 2cn log₂ n − 2cn + 4n ≤ cn log₂ n
      • The above inequality can be satisfied when c ≥ 4.
    • Conclusion: Therefore, T(n) ∈ O(n log n) if c ≥ 4.

    Another Example

    • Recurrence Relation: T(n) = T([n/2]) + T([n/2]) + 1
    • Claim: T(n) ∈ O(n)
    • Proof: Induction over n.
      • Hypothesis: T(k) ≤ ck for all k < n.
      • Using the recurrence relation and inductive hypothesis: T(n) = T([n/2]) + T([n/2]) + 1 ≤ c[n/2] + c[n/2] + 1 = cn + 1
    • Conclusion: This proves T(n)∈O(n)

    Recursion Tree Method

    • Example: T(n) = 3T(n/4) + n²
    • Analysis: The recursion tree method is used to analyze the time complexity of the recurrence relation. The table below gives an indication of the impact on different layers.
      Layer Node count (Layer) Impact (Layer)
      0 30 = 1 n2
      1 31 = 3 3(n/4)2
      i 3i (n/4)2
      log4 n nlog4 3
    • Conclusion: The recurrence is O(n2).

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    Description

    This quiz explores the recursion tree method for solving recurrence relations. It covers base cases, induction hypotheses, and the total number of nodes at each level of the tree. Ideal for students learning about algorithm complexity and recursive functions.

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