1706. Where Will the Ball Fall

class Solution {
public:
    vector<int> findBall(vector<vector<int>>& grid) {
        int n = grid.size();
        int m = grid[0].size();
        
        vector<int> ans(m);
        for (int i = 0; i < m; ++i)
            ans[i] = i;
        
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < m; ++j) {
                int cur = ans[j];
                if (cur == -1) continue;
                
                int next = cur + grid[i][cur];
                if (next < 0 || m == next || grid[i][next] != grid[i][cur]) {
                    ans[j] = -1;
                    continue;
                }
                
                ans[j] = next;
            }
        }
        
        return ans;
    }
};