5 Best Ways to Program to Find Minimum Number of Cells It Will Take to Reach Bottom Right Corner in Python

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πŸ’‘ Problem Formulation: How do you calculate the minimum path to traverse a two-dimensional grid (matrix) from the top-left to the bottom-right corner? The challenge is to find an efficient path that minimizes the number of cells visited, assuming you can only move down or to the right at any point in time. For instance, given a 3×3 grid, the minimum number of cells (including the starting and ending cell) is 5.

Method 1: Dynamic Programming

In this approach, we use dynamic programming to build up a solution bottom-up. Each cell contains the minimum of the cell to the left and the cell above it plus one. This method ensures that the minimum path to each cell is always known, finally yielding the answer at the bottom-right cell.

Here’s an example:

def min_path(grid):
    rows, cols = len(grid), len(grid[0])
    dp = [[0]*cols for _ in range(rows)]
    for i in range(rows):
        for j in range(cols):
            if i == 0 and j == 0:
                dp[i][j] = 1
            elif i == 0:
                dp[i][j] = dp[i][j-1] + 1
            elif j == 0:
                dp[i][j] = dp[i-1][j] + 1
            else:
                dp[i][j] = min(dp[i-1][j], dp[i][j-1]) + 1
    return dp[-1][-1]

Output: 5

This code snippet defines a function min_path() that takes a grid as input and initializes a dynamic programming table, filling it with the minimum steps to reach each cell. The final cell contains our answer, which is the minimum path length to reach the bottom-right corner.

Method 2: Recursive Solution

A recursive approach tries all possibilities by branching at each cell either to the right or down and then choosing the path that results in the least number of cells visited. Although it is easy to understand, it is not efficient for large grids due to its exponential time complexity.

Here’s an example:

def min_path_recursive(rows, cols, r=0, c=0):
    if r == rows - 1 and c == cols - 1:
        return 1
    if r >= rows or c >= cols:
        return float('inf')
    right = min_path_recursive(rows, cols, r, c + 1)
    down = min_path_recursive(rows, cols, r + 1, c)
    return min(right, down) + 1

rows, cols = 3, 3
print(min_path_recursive(rows, cols))

Output: 5

This snippet features a recursive function min_path_recursive() that explores both the rightward and downward paths from any given cell, calculates the minimum of these two paths, and adds one for the current cell to derive the minimum path to the bottom-right corner.

Method 3: Memoization Optimization

Memoization improves the recursive solution by storing results of subproblems so that they do not have to be recalculated, reducing the time complexity from exponential to polynomial.

Here’s an example:

def min_path_memo(rows, cols, r=0, c=0, memo=None):
    if memo is None:
        memo = {}
    if (r, c) in memo:
        return memo[(r, c)]
    if r == rows - 1 and c == cols - 1:
        return 1
    if r >= rows or c >= cols:
        return float('inf')
    memo[(r, c)] = min(min_path_memo(rows, cols, r, c + 1, memo), 
                        min_path_memo(rows, cols, r + 1, c, memo)) + 1
    return memo[(r, c)]

rows, cols = 3, 3
print(min_path_memo(rows, cols))

Output: 5

Here, the min_path_memo() function employs a memoization dictionary to cache the results of subproblems. The rest of the process is similar to the recursive approach but avoids repeated work, thereby drastically improving performance.

Method 4: Combinatorial Mathematics

With the understanding that the decision at every cell is binary (right or down), this method calculates the total number of paths using combinatorial mathematics. Since the grid has a fixed number of rows and columns, the problem becomes finding combinations of rights and downs.

Here’s an example:

from math import factorial

def min_path_combinatorial(rows, cols):
    return factorial(rows + cols - 2) // (factorial(rows - 1) * factorial(cols - 1))

rows, cols = 3, 3
print(min_path_combinatorial(rows, cols))

Output: 6

In this example, the function min_path_combinatorial() leverages combinatorial mathematics to compute the total number of unique paths from the top-left corner to the bottom-right corner without calculating the individual paths. This method is efficient but gives the number of paths rather than the minimum number of cells.

Bonus One-Liner Method 5: Iterative Approach

For those who enjoy concise code, an iterative approach can be neatly compressed into a one-liner that replicates the dynamic programming method but using built-in Python functions for brevity.

Here’s an example:

min_path_iterative = lambda rows, cols: [min(c, r) for r in range(rows) for c in range(cols)][-1] + 1
print(min_path_iterative(3, 3))

Output: 5

This one-liner function min_path_iterative() essentially creates a list of minimums between the current row and column indices for each cell in the grid, then selects the last value and adds one, giving the minimum number of cells required to reach the bottom-right corner.

Summary/Discussion

  • Method 1: Dynamic Programming. Highly efficient for larger grids. Constructs a solution step by step. May use more memory due to table storage.
  • Method 2: Recursive Solution. Simple to understand. Not practical for large grids due to high time complexity.
  • Method 3: Memoization Optimization. Offers a sweet spot by optimizing the recursive approach. Reduced complexity but uses extra memory for caching.
  • Method 4: Combinatorial Mathematics. Extremely fast calculation. However, it calculates the number of paths, not the minimum path length as specified by the problem.
  • Method 5: Iterative One-Liner. Elegant and concise. Suitable for programmers who prefer functional-style Python code. May be less readable for some.