Shortest path in a Binary Maze

Given a MxN matrix where each element can either be 0 or 1. We need to find the shortest path between a given source cell to a destination cell. The path can only be created out of a cell if its value is 1.
Expected time complexity is O(MN).
For example – 

Input:
mat[ROW][COL]  = {{1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
                  {1, 0, 1, 0, 1, 1, 1, 0, 1, 1 },
                  {1, 1, 1, 0, 1, 1, 0, 1, 0, 1 },
                  {0, 0, 0, 0, 1, 0, 0, 0, 0, 1 },
                  {1, 1, 1, 0, 1, 1, 1, 0, 1, 0 },
                  {1, 0, 1, 1, 1, 1, 0, 1, 0, 0 },
                  {1, 0, 0, 0, 0, 0, 0, 0, 0, 1 },
                  {1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
                  {1, 1, 0, 0, 0, 0, 1, 0, 0, 1 }};
Source = {0, 0};
Destination = {3, 4};

Output:
Shortest Path is 11 

The idea is inspired from Lee algorithm and uses BFS.  

  1. We start from the source cell and calls BFS procedure.
  2. We maintain a queue to store the coordinates of the matrix and initialize it with the source cell.
  3. We also maintain a Boolean array visited of same size as our input matrix and initialize all its elements to false.
    1. We LOOP till queue is not empty
    2. Dequeue front cell from the queue
    3. Return if the destination coordinates have reached.
    4. For each of its four adjacent cells, if the value is 1 and they are not visited yet, we enqueue it in the queue and also mark them as visited.

Note that BFS works here because it doesn’t consider a single path at once. It considers all the paths starting from the source and moves ahead one unit in all those paths at the same time which makes sure that the first time when the destination is visited, it is the shortest path.
Below is the implementation of the idea –  

C++

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// C++ program to find the shortest path between
// a given source cell to a destination cell.
#include <bits/stdc++.h>
using namespace std;
#define ROW 9
#define COL 10
 
//To store matrix cell cordinates
struct Point
{
    int x;
    int y;
};
 
// A Data Structure for queue used in BFS
struct queueNode
{
    Point pt;  // The cordinates of a cell
    int dist;  // cell's distance of from the source
};
 
// check whether given cell (row, col) is a valid
// cell or not.
bool isValid(int row, int col)
{
    // return true if row number and column number
    // is in range
    return (row >= 0) && (row < ROW) &&
           (col >= 0) && (col < COL);
}
 
// These arrays are used to get row and column
// numbers of 4 neighbours of a given cell
int rowNum[] = {-1, 0, 0, 1};
int colNum[] = {0, -1, 1, 0};
 
// function to find the shortest path between
// a given source cell to a destination cell.
int BFS(int mat[][COL], Point src, Point dest)
{
    // check source and destination cell
    // of the matrix have value 1
    if (!mat[src.x][src.y] || !mat[dest.x][dest.y])
        return -1;
 
    bool visited[ROW][COL];
    memset(visited, false, sizeof visited);
     
    // Mark the source cell as visited
    visited[src.x][src.y] = true;
 
    // Create a queue for BFS
    queue<queueNode> q;
     
    // Distance of source cell is 0
    queueNode s = {src, 0};
    q.push(s);  // Enqueue source cell
 
    // Do a BFS starting from source cell
    while (!q.empty())
    {
        queueNode curr = q.front();
        Point pt = curr.pt;
 
        // If we have reached the destination cell,
        // we are done
        if (pt.x == dest.x && pt.y == dest.y)
            return curr.dist;
 
        // Otherwise dequeue the front
        // cell in the queue
        // and enqueue its adjacent cells
        q.pop();
 
        for (int i = 0; i < 4; i++)
        {
            int row = pt.x + rowNum[i];
            int col = pt.y + colNum[i];
             
            // if adjacent cell is valid, has path and
            // not visited yet, enqueue it.
            if (isValid(row, col) && mat[row][col] &&
               !visited[row][col])
            {
                // mark cell as visited and enqueue it
                visited[row][col] = true;
                queueNode Adjcell = { {row, col},
                                      curr.dist + 1 };
                q.push(Adjcell);
            }
        }
    }
 
    // Return -1 if destination cannot be reached
    return -1;
}
 
// Driver program to test above function
int main()
{
    int mat[ROW][COL] =
    {
        { 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
        { 1, 0, 1, 0, 1, 1, 1, 0, 1, 1 },
        { 1, 1, 1, 0, 1, 1, 0, 1, 0, 1 },
        { 0, 0, 0, 0, 1, 0, 0, 0, 0, 1 },
        { 1, 1, 1, 0, 1, 1, 1, 0, 1, 0 },
        { 1, 0, 1, 1, 1, 1, 0, 1, 0, 0 },
        { 1, 0, 0, 0, 0, 0, 0, 0, 0, 1 },
        { 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
        { 1, 1, 0, 0, 0, 0, 1, 0, 0, 1 }
    };
 
    Point source = {0, 0};
    Point dest = {3, 4};
 
    int dist = BFS(mat, source, dest);
 
    if (dist != -1)
        cout << "Shortest Path is " << dist ;
    else
        cout << "Shortest Path doesn't exist";
 
    return 0;
}

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Java

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// Java program to find the shortest
// path between a given source cell
// to a destination cell.
import java.util.*;
 
class GFG
{
static int ROW = 9;
static int COL = 10;
 
// To store matrix cell cordinates
static class Point
{
    int x;
    int y;
 
    public Point(int x, int y)
    {
        this.x = x;
        this.y = y;
    }
};
 
// A Data Structure for queue used in BFS
static class queueNode
{
    Point pt; // The cordinates of a cell
    int dist; // cell's distance of from the source
 
    public queueNode(Point pt, int dist)
    {
        this.pt = pt;
        this.dist = dist;
    }
};
 
// check whether given cell (row, col)
// is a valid cell or not.
static boolean isValid(int row, int col)
{
    // return true if row number and
    // column number is in range
    return (row >= 0) && (row < ROW) &&
           (col >= 0) && (col < COL);
}
 
// These arrays are used to get row and column
// numbers of 4 neighbours of a given cell
static int rowNum[] = {-1, 0, 0, 1};
static int colNum[] = {0, -1, 1, 0};
 
// function to find the shortest path between
// a given source cell to a destination cell.
static int BFS(int mat[][], Point src,
                            Point dest)
{
    // check source and destination cell
    // of the matrix have value 1
    if (mat[src.x][src.y] != 1 ||
        mat[dest.x][dest.y] != 1)
        return -1;
 
    boolean [][]visited = new boolean[ROW][COL];
     
    // Mark the source cell as visited
    visited[src.x][src.y] = true;
 
    // Create a queue for BFS
    Queue<queueNode> q = new LinkedList<>();
     
    // Distance of source cell is 0
    queueNode s = new queueNode(src, 0);
    q.add(s); // Enqueue source cell
 
    // Do a BFS starting from source cell
    while (!q.isEmpty())
    {
        queueNode curr = q.peek();
        Point pt = curr.pt;
 
        // If we have reached the destination cell,
        // we are done
        if (pt.x == dest.x && pt.y == dest.y)
            return curr.dist;
 
        // Otherwise dequeue the front cell
        // in the queue and enqueue
        // its adjacent cells
        q.remove();
 
        for (int i = 0; i < 4; i++)
        {
            int row = pt.x + rowNum[i];
            int col = pt.y + colNum[i];
             
            // if adjacent cell is valid, has path
            // and not visited yet, enqueue it.
            if (isValid(row, col) &&
                    mat[row][col] == 1 &&
                    !visited[row][col])
            {
                // mark cell as visited and enqueue it
                visited[row][col] = true;
                queueNode Adjcell = new queueNode
                             (new Point(row, col),
                                   curr.dist + 1 );
                q.add(Adjcell);
            }
        }
    }
 
    // Return -1 if destination cannot be reached
    return -1;
}
 
// Driver Code
public static void main(String[] args)
{
    int mat[][] = {{ 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
                   { 1, 0, 1, 0, 1, 1, 1, 0, 1, 1 },
                   { 1, 1, 1, 0, 1, 1, 0, 1, 0, 1 },
                   { 0, 0, 0, 0, 1, 0, 0, 0, 0, 1 },
                   { 1, 1, 1, 0, 1, 1, 1, 0, 1, 0 },
                   { 1, 0, 1, 1, 1, 1, 0, 1, 0, 0 },
                   { 1, 0, 0, 0, 0, 0, 0, 0, 0, 1 },
                   { 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
                   { 1, 1, 0, 0, 0, 0, 1, 0, 0, 1 }};
 
    Point source = new Point(0, 0);
    Point dest = new Point(3, 4);
 
    int dist = BFS(mat, source, dest);
 
    if (dist != -1)
        System.out.println("Shortest Path is " + dist);
    else
        System.out.println("Shortest Path doesn't exist");
    }
}
 
// This code is contributed by PrinciRaj1992

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Python

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# Python program to find the shortest
# path between a given source cell
# to a destination cell.
 
from collections import deque
ROW = 9
COL = 10
 
# To store matrix cell cordinates
class Point:
    def __init__(self,x: int, y: int):
        self.x = x
        self.y = y
 
# A data structure for queue used in BFS
class queueNode:
    def __init__(self,pt: Point, dist: int):
        self.pt = pt  # The cordinates of the cell
        self.dist = dist  # Cell's distance from the source
 
# Check whether given cell(row,col)
# is a valid cell or not
def isValid(row: int, col: int):
    return (row >= 0) and (row < ROW) and
                   (col >= 0) and (col < COL)
 
# These arrays are used to get row and column
# numbers of 4 neighbours of a given cell
rowNum = [-1, 0, 0, 1]
colNum = [0, -1, 1, 0]
 
# Function to find the shortest path between
# a given source cell to a destination cell.
def BFS(mat, src: Point, dest: Point):
     
    # check source and destination cell
    # of the matrix have value 1
    if mat[src.x][src.y]!=1 or mat[dest.x][dest.y]!=1:
        return -1
     
    visited = [[False for i in range(COL)]
                       for j in range(ROW)]
     
    # Mark the source cell as visited
    visited[src.x][src.y] = True
     
    # Create a queue for BFS
    q = deque()
     
    # Distance of source cell is 0
    s = queueNode(src,0)
    q.append(s) #  Enqueue source cell
     
    # Do a BFS starting from source cell
    while q:
 
        curr = q.popleft() # Dequeue the front cell
         
        # If we have reached the destination cell,
        # we are done
        pt = curr.pt
        if pt.x == dest.x and pt.y == dest.y:
            return curr.dist
         
        # Otherwise enqueue its adjacent cells
        for i in range(4):
            row = pt.x + rowNum[i]
            col = pt.y + colNum[i]
             
            # if adjacent cell is valid, has path 
            # and not visited yet, enqueue it.
            if (isValid(row,col) and
               mat[row][col] == 1 and
                not visited[row][col]):
                visited[row][col] = True
                Adjcell = queueNode(Point(row,col),
                                    curr.dist+1)
                q.append(Adjcell)
     
    # Return -1 if destination cannot be reached
    return -1
 
# Driver code
def main():
    mat = [[ 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 ],
           [ 1, 0, 1, 0, 1, 1, 1, 0, 1, 1 ],
           [ 1, 1, 1, 0, 1, 1, 0, 1, 0, 1 ],
           [ 0, 0, 0, 0, 1, 0, 0, 0, 0, 1 ],
           [ 1, 1, 1, 0, 1, 1, 1, 0, 1, 0 ],
           [ 1, 0, 1, 1, 1, 1, 0, 1, 0, 0 ],
           [ 1, 0, 0, 0, 0, 0, 0, 0, 0, 1 ],
           [ 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 ],
           [ 1, 1, 0, 0, 0, 0, 1, 0, 0, 1 ]]
    source = Point(0,0)
    dest = Point(3,4)
     
    dist = BFS(mat,source,dest)
     
    if dist!=-1:
        print("Shortest Path is",dist)
    else:
        print("Shortest Path doesn't exist")
main()
 
# This code is contributed by stutipathak31jan

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C#

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// C# program to find the shortest
// path between a given source cell
// to a destination cell.
using System;
using System.Collections.Generic;
 
class GFG
{
static int ROW = 9;
static int COL = 10;
 
// To store matrix cell cordinates
public class Point
{
    public int x;
    public int y;
 
    public Point(int x, int y)
    {
        this.x = x;
        this.y = y;
    }
};
 
// A Data Structure for queue used in BFS
public class queueNode
{
    // The cordinates of a cell
    public Point pt;
     
    // cell's distance of from the source
    public int dist;
 
    public queueNode(Point pt, int dist)
    {
        this.pt = pt;
        this.dist = dist;
    }
};
 
// check whether given cell (row, col)
// is a valid cell or not.
static bool isValid(int row, int col)
{
    // return true if row number and
    // column number is in range
    return (row >= 0) && (row < ROW) &&
           (col >= 0) && (col < COL);
}
 
// These arrays are used to get row and column
// numbers of 4 neighbours of a given cell
static int []rowNum = {-1, 0, 0, 1};
static int []colNum = {0, -1, 1, 0};
 
// function to find the shortest path between
// a given source cell to a destination cell.
static int BFS(int [,]mat, Point src,
                           Point dest)
{
    // check source and destination cell
    // of the matrix have value 1
    if (mat[src.x, src.y] != 1 ||
        mat[dest.x, dest.y] != 1)
        return -1;
 
    bool [,]visited = new bool[ROW, COL];
     
    // Mark the source cell as visited
    visited[src.x, src.y] = true;
 
    // Create a queue for BFS
    Queue<queueNode> q = new Queue<queueNode>();
     
    // Distance of source cell is 0
    queueNode s = new queueNode(src, 0);
    q.Enqueue(s); // Enqueue source cell
 
    // Do a BFS starting from source cell
    while (q.Count != 0)
    {
        queueNode curr = q.Peek();
        Point pt = curr.pt;
 
        // If we have reached the destination cell,
        // we are done
        if (pt.x == dest.x && pt.y == dest.y)
            return curr.dist;
 
        // Otherwise dequeue the front cell
        // in the queue and enqueue
        // its adjacent cells
        q.Dequeue();
 
        for (int i = 0; i < 4; i++)
        {
            int row = pt.x + rowNum[i];
            int col = pt.y + colNum[i];
             
            // if adjacent cell is valid, has path
            // and not visited yet, enqueue it.
            if (isValid(row, col) &&
                    mat[row, col] == 1 &&
               !visited[row, col])
            {
                // mark cell as visited and enqueue it
                visited[row, col] = true;
                queueNode Adjcell = new queueNode
                           (new Point(row, col),
                                curr.dist + 1 );
                q.Enqueue(Adjcell);
            }
        }
    }
 
    // Return -1 if destination cannot be reached
    return -1;
}
 
// Driver Code
public static void Main(String[] args)
{
    int [,]mat = {{ 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
                  { 1, 0, 1, 0, 1, 1, 1, 0, 1, 1 },
                   { 1, 1, 1, 0, 1, 1, 0, 1, 0, 1 },
                  { 0, 0, 0, 0, 1, 0, 0, 0, 0, 1 },
                  { 1, 1, 1, 0, 1, 1, 1, 0, 1, 0 },
                  { 1, 0, 1, 1, 1, 1, 0, 1, 0, 0 },
                  { 1, 0, 0, 0, 0, 0, 0, 0, 0, 1 },
                  { 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 },
                  { 1, 1, 0, 0, 0, 0, 1, 0, 0, 1 }};
 
    Point source = new Point(0, 0);
    Point dest = new Point(3, 4);
 
    int dist = BFS(mat, source, dest);
 
    if (dist != -1)
        Console.WriteLine("Shortest Path is " + dist);
    else
        Console.WriteLine("Shortest Path doesn't exist");
    }
}
 
// This code is contributed by PrinciRaj1992

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Output : 

Shortest Path is 11

This article is contributed by Aditya Goel. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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