GATE CS 2026 Set 2 — Question 39
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Algorithms → Dynamic Programming → Memoization vs Tabulation
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Question
Consider a table , where the elements , represent the cost of the optimal solutions of different subproblems of a problem that is being solved using a dynamic programming algorithm. The recursive formulation to compute the table entries is as follows:Consider the following two algorithms to compute entries of . Assume that for both the algorithms, for all , has been initialized to 1.Algorithm :
Algorithm :
Algorithm is said to be correct if and only if it calculates the correct values of , for all , (as per the recursive formulation) at the end of the execution of the algorithm .Which one of the following statements is true?
For i = 1, 2, ..., n
For j = 1, 2, ..., n
T[i][j] = 2T[i-1][j] + 3T[i][j-1]
For s = 2, 3, ..., 2n
For i = 1, 2, ..., n
For j = 1, 2, ..., n
If (i + j == s)
T[i][j] = 2T[i-1][j] + 3T[i][j-1]
Correct answer
(A) Both algorithms B₁ and B₂ are correct
Solution
The recurrence relation is . This means to compute , we need the values of the cell directly above it () and the cell directly to its left ().Algorithm (Row-Major Order):
- It iterates from 1 to and from 1 to .
- When computing , the row index has already been fully processed (since the outer loop is on ), so is available.
- The column index in the current row has already been processed (since the inner loop is on ), so is available.
- Thus, respects the dependencies and is correct.
- It iterates on the sum of indices from 2 to .
- When computing where , the dependencies are and .
- The sum of indices for is .
- The sum of indices for is .
- Since the outer loop iterates on in increasing order, all cells with index sum have been computed in the previous iteration of the outer loop.
- Thus, also respects the dependencies and is correct.
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