1706. Where Will the Ball Fall

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.

• A board that redirects the ball to the right spans the top-left corner to the bottom-right corner and is represented in the grid as 1.
• A board that redirects the ball to the left spans the top-right corner to the bottom-left corner and is represented in the grid as -1.

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.

class Solution {
    int helper(vector<vector<int>>& grid, int x) {
        int n = grid.size(), m = grid[0].size();
        for(auto g : grid) {
            int next = g[x] + x;
            if(next == -1 or next == m) return -1;
            if(g[next] != g[x]) return -1;
            x = next;
        }
        return x;
    }
public:
    vector<int> findBall(vector<vector<int>>& grid) {
        vector<int> res;
        int m = grid[0].size();
        for(int i = 0; i < m; i++)
            res.push_back(helper(grid,i));
        return res;
    }
};