Shortest distance between two cells in a matrix or grid

Given a matrix of N*M order. Find the shortest distance from a source cell to a destination cell, traversing through limited cells only. Also you can move only up, down, left and right. If found output the distance else -1.
s represents ‘source’
d represents ‘destination’
* represents cell you can travel
0 represents cell you can not travel
This problem is meant for single source and destination.

Examples:

Input : {'0', '*', '0', 's'},
        {'*', '0', '*', '*'},
        {'0', '*', '*', '*'},
        {'d', '*', '*', '*'}
Output : 6

Input :  {'0', '*', '0', 's'},
         {'*', '0', '*', '*'},
         {'0', '*', '*', '*'},
         {'d', '0', '0', '0'}
Output :  -1

The idea is to BFS (breadth first search) on matrix cells. Note that we can always use BFS to find shortest path if graph is unweighted.



  1. Store each cell as a node with their row, column values and distance from source cell.
  2. Start BFS with source cell.
  3. Make a visited array with all having “false” values except ‘0’cells which are assigned “true” values as they can not be traversed.
  4. Keep updating distance from source value in each move.
  5. Return distance when destination is met, else return -1 (no path exists in between source and destination).
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// C++ Code implementation for above problem
#include <bits/stdc++.h>
using namespace std;
  
#define N 4
#define M 4
  
// QItem for current location and distance
// from source location
class QItem {
public:
    int row;
    int col;
    int dist;
    QItem(int x, int y, int w)
        : row(x), col(y), dist(w)
    {
    }
};
  
int minDistance(char grid[N][M])
{
    QItem source(0, 0, 0);
  
    // To keep track of visited QItems. Marking
    // blocked cells as visited.
    bool visited[N][M];
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < M; j++)
        {
            if (grid[i][j] == '0')
                visited[i][j] = true;
            else
                visited[i][j] = false;
  
            // Finding source
            if (grid[i][j] == 's')
            {
               source.row = i;
               source.col = j;
            }
        }
    }
  
    // applying BFS on matrix cells starting from source
    queue<QItem> q;
    q.push(source);
    visited[source.row][source.col] = true;
    while (!q.empty()) {
        QItem p = q.front();
        q.pop();
  
        // Destination found;
        if (grid[p.row][p.col] == 'd')
            return p.dist;
  
        // moving up
        if (p.row - 1 >= 0 &&
            visited[p.row - 1][p.col] == false) {
            q.push(QItem(p.row - 1, p.col, p.dist + 1));
            visited[p.row - 1][p.col] = true;
        }
  
        // moving down
        if (p.row + 1 < N &&
            visited[p.row + 1][p.col] == false) {
            q.push(QItem(p.row + 1, p.col, p.dist + 1));
            visited[p.row + 1][p.col] = true;
        }
  
        // moving left
        if (p.col - 1 >= 0 &&
            visited[p.row][p.col - 1] == false) {
            q.push(QItem(p.row, p.col - 1, p.dist + 1));
            visited[p.row][p.col - 1] = true;
        }
  
         // moving right
        if (p.col + 1 < M &&
            visited[p.row][p.col + 1] == false) {
            q.push(QItem(p.row, p.col + 1, p.dist + 1));
            visited[p.row][p.col + 1] = true;
        }
    }
    return -1;
}
  
// Driver code
int main()
{
    char grid[N][M] = { { '0', '*', '0', 's' },
                        { '*', '0', '*', '*' },
                        { '0', '*', '*', '*' },
                        { 'd', '*', '*', '*' } };
  
    cout << minDistance(grid);
    return 0;
}

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Output:

6

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