Longest Possible Route in a Matrix with Hurdles

3.5

Given an M x N matrix, with a few hurdles arbitrarily placed, calculate the length of longest possible route possible from source to destination within the matrix. We are allowed to move to only adjacent cells which are not hurdles. The route cannot contains any diagonal moves and a location once visited in a particular path cannot be visited again.

For example, longest path with no hurdles from source to destination is highlighted for below matrix. The length of the path is 24.

Matrix

The idea is to use Backtracking. We start from the source cell of the matrix, move forward in all four allowed directions and recursively checks if they leads to the solution or not. If destination is found, we update the value of longest path else if none of the above solutions work we return false from our function.

Below is C++ implementation of above idea –

// C++ program to find Longest Possible Route in a
// matrix with hurdles
#include <bits/stdc++.h>
using namespace std;
#define R 3
#define C 10

// A Pair to store status of a cell. found is set to
// true of destination is reachable and value stores
// distance of longest path
struct Pair
{
    // true if destination is found
    bool found;

    // stores cost of longest path from current cell to
    // destination cell
    int value;
};

// Function to find Longest Possible Route in the
// matrix with hurdles. If the destination is not reachable
// the function returns false with cost INT_MAX.
// (i, j) is source cell and (x, y) is destination cell.
Pair findLongestPathUtil(int mat[R][C], int i, int j,
    int x, int y, bool visited[R][C])
{

    // if (i, j) itself is destination, return true
    if (i == x && j == y)
    {
        Pair p = { true, 0 };
        return p;
    }

    // if not a valid cell, return false
    if (i < 0 || i >= R || j < 0 || j >= C ||
            mat[i][j] == 0 || visited[i][j])
    {
        Pair p = { false, INT_MAX };
        return p;
    }

    // include (i, j) in current path i.e.
    // set visited(i, j) to true
    visited[i][j] = true;

    // res stores longest path from current cell (i, j) to
    // destination cell (x, y)
    int res = INT_MIN;

    // go left from current cell
    Pair sol = findLongestPathUtil(mat, i, j - 1, x, y, visited);

    // if destination can be reached on going left from current
    // cell, update res
    if (sol.found)
        res = max(res, sol.value);

    // go right from current cell
    sol = findLongestPathUtil(mat, i, j + 1, x, y, visited);

    // if destination can be reached on going right from current
    // cell, update res
    if (sol.found)
        res = max(res, sol.value);

    // go up from current cell
    sol = findLongestPathUtil(mat, i - 1, j, x, y, visited);

    // if destination can be reached on going up from current
    // cell, update res
    if (sol.found)
        res = max(res, sol.value);

    // go down from current cell
    sol = findLongestPathUtil(mat, i + 1, j, x, y, visited);

    // if destination can be reached on going down from current
    // cell, update res
    if (sol.found)
        res = max(res, sol.value);

    // Backtrack
    visited[i][j] = false;

    // if destination can be reached from current cell,
    // return true
    if (res != INT_MIN)
    {
        Pair p = { true, 1 + res };
        return p;
    }

    // if destination can't be reached from current cell,
    // return false
    else
    {
        Pair p = { false, INT_MAX };
        return p;
    }
}

// A wrapper function over findLongestPathUtil()
void findLongestPath(int mat[R][C], int i, int j, int x,
                                                  int y)
{
    // create a boolean matrix to store info about
    // cells already visited in current route
    bool visited[R][C];

    // initailize visited to false
    memset(visited, false, sizeof visited);

    // find longest route from (i, j) to (x, y) and
    // print its maximum cost
    Pair p = findLongestPathUtil(mat, i, j, x, y, visited);
    if (p.found)
        cout << "Length of longest possible route is "
             << p.value;

    // If the destination is not reachable
    else
        cout << "Destination not reachable from given source";
}

// Driver code
int main()
{
    // input matrix with hurdles shown with number 0
    int mat[R][C] =
    {
        { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 },
        { 1, 1, 0, 1, 1, 0, 1, 1, 0, 1 },
        { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 }
    };

    // find longest path with source (0, 0) and
    // destination (1, 7)
    findLongestPath(mat, 0, 0, 1, 7);

    return 0;
}

Output:

Length of longest possible route is 24

This article is contributed by Aditya Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

GATE CS Corner    Company Wise Coding Practice

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.

Recommended Posts:



3.5 Average Difficulty : 3.5/5.0
Based on 11 vote(s)










Writing code in comment? Please use ide.geeksforgeeks.org, generate link and share the link here.