Check if possible to cross the matrix with given power

Given a matrix of N X M, each cell consist of an integer. We have initial power of K and we are allowed to move right, down or diagonal. When we move to any cell, we absorb mat[i][j] value and loose that much amount from your power. If our power became less than 0 at any time, we cannot move further from that point. Now, our task is to find if there is any path from (1, 1) to (n, m) that we can cover with power k. If possible, output the maximum value we can absorb, else if there is no path print “-1”.

Examples :

Input : N = 3, M = 3, K = 7
        mat[][] = { { 2, 3, 1 },
                    { 6, 1, 9 },
                    { 8, 2, 3 } };
Output : 6
Path (1, 1) -> (2, 2) -> (3, 3) to complete
journey to absorb 6 value.

Input : N = 3, M = 4, K = 9
        mat[][] = { 
                    { 2, 3, 4, 1 },
                    { 6, 5, 5, 3 },
                    { 5, 2, 3, 4 }
                  };
Output : -1

The idea is to use Dynamic Programming to solve the problem.
Approach :

  • Declare a boolean 3D matrix, say dp[ ][ ][ ], with N*M*(K+1) dimension such that dp[ i ][ j ][ k ] is true if it possible to reach the square in the ith row and jth column with exactly k value collected so far.
  • We can write the recurrence dp[ i ][ j ][ k ] = true if either
    dp[i-1][j][k-mat[i][j]] or 
    dp[i][j-1][k-mat[i][j]] or 
    dp[i-1][j-1][k-mat[i][j]] 

    i.e the three possible moves we could have.

  • We have base case dp[0][0][0] be true.
  • The answer is -2 if dp[n-1][m-1][k] is false for all k between 0 and k+1.
  • Otherwise, the answer is the maximum k such that dp[n-1][m-1][k] is true.

Below is C++ implementation of this approach :

// CPP program to find if it is possible to cross
// the matrix with given power
#include <bits/stdc++.h>
#define N 105
#define R 3
#define C 4
using namespace std;

int maximumValue(int n, int m, int p, int grid[R][C])
{
    bool dp[N][N][N];

    // Initializing array dp with false value.
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            for (int k = 0; k < N; k++)
                dp[i][j][k] = false;
        }
    }

    // For each value of dp[i][j][k]
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            for (int k = grid[i][j]; k <= p; k++) {

                // For first cell and for each value of k
                if (i == 0 && j == 0) {
                    if (k == grid[i][j])
                        dp[i][j][k] = true;
                }

                // For first cell of each row
                else if (i == 0) {
                    dp[i][j][k] = (dp[i][j][k] || 
                        dp[i][j - 1][k - grid[i][j]]);
                }

                // For first cell of each column
                else if (j == 0) {
                    dp[i][j][k] = (dp[i][j][k] ||
                        dp[i - 1][j][k - grid[i][j]]);
                }

                // For rest of the cell
                else {

                    // Down movement.
                    dp[i][j][k] = (dp[i][j][k] ||
                        dp[i][j - 1][k - grid[i][j]]);

                    // Right movement.
                    dp[i][j][k] = (dp[i][j][k] ||
                        dp[i - 1][j][k - grid[i][j]]);

                    // Diagonal movement.
                    dp[i][j][k] = (dp[i][j][k] ||
                        dp[i - 1][j - 1][k - grid[i][j]]);
                }
            }
        }
    }

    // Finding maximum k.
    int ans = 0;
    for (ans = k; ans >= 0; ans--)
        if (dp[n - 1][m - 1][ans])
            break;

    return ans;
}

// Driver Code
int main()
{
    int n = 3, m = 4, p = 9;
    int grid[R][C] = {
        { 2, 3, 4, 1 },
        { 6, 5, 5, 3 },
        { 5, 2, 3, 4 }
    };

    cout << maximumValue(n, m, p, grid) << endl;
    return 0;
}


Output:

-1

Time Complexity: O(n3)

This article is contributed by anuj0503. 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.

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