LeetCode //C - 1926. Nearest Exit from Entrance in Maze

1926. Nearest Exit from Entrance in Maze

You are given an m x n matrix maze (0-indexed) with empty cells (represented as '.') and walls (represented as '+'). You are also given the entrance of the maze, where entrance = [entrancerow, entrancecol] denotes the row and column of the cell you are initially standing at.

In one step, you can move one cell up, down, left, or right. You cannot step into a cell with a wall, and you cannot step outside the maze. Your goal is to find the nearest exit from the entrance. An exit is defined as an empty cell that is at the border of the maze. The entrance does not count as an exit.

Return the number of steps in the shortest path from the entrance to the nearest exit, or -1 if no such path exists.

Example 1:

Input: maze = [["+","+",".","+"],[".",".",".","+"],["+","+","+","."]], entrance = [1,2]
Output: 1
Explanation: There are 3 exits in this maze at [1,0], [0,2], and [2,3].

Initially, you are at the entrance cell [1,2].

• You can reach [1,0] by moving 2 steps left.

• You can reach [0,2] by moving 1 step up.

It is impossible to reach [2,3] from the entrance.

Thus, the nearest exit is [0,2], which is 1 step away.

Example 2:

Input: maze = [["+","+","+"],[".",".","."],["+","+","+"]], entrance = [1,0]
Output: 2
Explanation: There is 1 exit in this maze at [1,2].

[1,0] does not count as an exit since it is the entrance cell.

Initially, you are at the entrance cell [1,0].

• You can reach [1,2] by moving 2 steps right.

Thus, the nearest exit is [1,2], which is 2 steps away.

Example 3:

Input: maze = [[".","+"]], entrance = [0,0]
Output: -1
Explanation: There are no exits in this maze.

Constraints:

• maze.length == m
• maze[i].length == n
• 1 <= m, n <= 100
• maze[i][j] is either '.' or '+'.
• entrance.length == 2
• 0 <= entrancerow < m
• 0 <= entrancecol < n
• entrance will always be an empty cell.

From: LeetCode

Link: 1926. Nearest Exit from Entrance in Maze

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Solution:

Ideas：

This function uses Breadth-First Search (BFS) to find the nearest exit. It checks all possible paths from the entrance and returns the number of steps to the nearest exit. If no exit is reachable, it returns -1. Please note that this code should be part of a complete C program and compiled with a C compiler to run. It assumes that the maze is properly allocated and passed to the function along with the size parameters and entrance coordinates.

Code:

// Helper function to check if the current position is valid and not a wall.
int isValid(char **maze, int x, int y, int m, int n) {
    if (x >= 0 && x < m && y >= 0 && y < n && maze[x][y] == '.')
        return 1;
    return 0;
}

// Helper function to check if the current position is at the border and not the entrance.
int isExit(char **maze, int x, int y, int m, int n, int* entrance) {
    if ((x == 0 || x == m - 1 || y == 0 || y == n - 1) && (x != entrance[0] || y != entrance[1]))
        return 1;
    return 0;
}

int nearestExit(char** maze, int mazeSize, int* mazeColSize, int* entrance, int entranceSize) {
    int directions[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}}; // Up, Down, Left, Right
    int m = mazeSize, n = mazeColSize[0];
    int queueSize = m * n;
    int queue[queueSize][3]; // Queue to store the x, y, and distance
    int front = 0, rear = 0;

    // Initialize the queue with the entrance position
    queue[rear][0] = entrance[0];
    queue[rear][1] = entrance[1];
    queue[rear][2] = 0;
    rear++;

    maze[entrance[0]][entrance[1]] = '+'; // Mark the entrance as visited

    while (front < rear) {
        // Dequeue the front element
        int x = queue[front][0];
        int y = queue[front][1];
        int dist = queue[front][2];
        front++;

        // Check if we've reached an exit
        if (isExit(maze, x, y, m, n, entrance)) {
            return dist;
        }

        // Check all four possible directions
        for (int i = 0; i < 4; i++) {
            int newX = x + directions[i][0];
            int newY = y + directions[i][1];

            // If valid, not a wall, and not visited, add to queue
            if (isValid(maze, newX, newY, m, n)) {
                maze[newX][newY] = '+'; // Mark as visited
                queue[rear][0] = newX;
                queue[rear][1] = newY;
                queue[rear][2] = dist + 1;
                rear++;
            }
        }
    }

    // If no exit is reached, return -1
    return -1;
}