You have a 2-D grid of size m x n representing a box, and you have n balls. The box is open on the top and bottom sides.

Each cell in the box has a diagonal board spanning two corners of the cell that can redirect a ball to the right or to the left.

We drop one ball at the top of each column of the box. Each ball can get stuck in the box or fall out of the bottom. A ball gets stuck if it hits a "V" shaped pattern between two boards or if a board redirects the ball into either wall of the box.

Return an array answer of size n where answer[i] is the column that the ball falls out of at the bottom after dropping the ball from the ith column at the top, or -1 if the ball gets stuck in the box.


Test Cases

Ball

Example 1:

Input: grid = [[1,1,1,-1,-1],[1,1,1,-1,-1],[-1,-1,-1,1,1],[1,1,1,1,-1],[-1,-1,-1,-1,-1]]
Output: [1,-1,-1,-1,-1]
Explanation: This example is shown in the photo.
Ball b0 is dropped at column 0 and falls out of the box at column 1.
Ball b1 is dropped at column 1 and will get stuck in the box between column 2 and 3 and row 1.
Ball b2 is dropped at column 2 and will get stuck on the box between column 2 and 3 and row 0.
Ball b3 is dropped at column 3 and will get stuck on the box between column 2 and 3 and row 0.
Ball b4 is dropped at column 4 and will get stuck on the box between column 2 and 3 and row 1.

Example 2:

Input: grid = [[1,1,1,1,1,1],[-1,-1,-1,-1,-1,-1],[1,1,1,1,1,1],[-1,-1,-1,-1,-1,-1]]
Output: [0,1,2,3,4,-1]

Solution

class Solution {
public:
    vector<int> findBall(vector<vector<int>>& grid) {
        int n = grid.size(), m = grid[0].size();
        vector<int> res;
        for(int j=0; j<m; j++) {
            int startCol = j;
            for(int i=0; i<n; i++) {
                int endCol = startCol + grid[i][startCol];
                if (endCol < 0 || endCol >= m || grid[i][endCol] != grid[i][startCol]) {
                    startCol = -1;
                    break;
                }
                startCol = endCol;
            }
            res.push_back(startCol);
        }
        return res;
    }
};
class Solution {
    public int[] findBall(int[][] grid) {
        int n = grid.length, m = grid[0].length;
        int[] res = new int[m];
        for(int j=0; j<m; j++) {
            int startCol = j;
            for(int i=0; i<n; i++) {
                int endCol = startCol + grid[i][startCol];
                if (endCol < 0 || endCol >= m || grid[i][endCol] != grid[i][startCol]) {
                    startCol = -1;
                    break;
                }
                startCol = endCol;
            }
            res[j] = startCol;
        }
        return res;
    }
}
Time Complexity: O(nm)
Space Complexity: O(m)