Where will the ball fall

Array Matrix Simulation

72.3% acceptance

Feb 25, 2026

3176

181

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.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn find_ball(grid: Vec<Vec<i32>>) -> Vec<i32> {
    let m = grid.len();
    let n = grid[0].len();
    (0..n as i32).map(|start| {
      let mut c = start;
      for r in 0..m {
        if grid[r][c as usize] == 1 {
          if c + 1 >= n as i32 || grid[r][(c + 1) as usize] == -1 {
            return -1;
          }
          c += 1;
        } else {
          if c - 1 < 0 || grid[r][(c - 1) as usize] == 1 {
            return -1;
          }
          c -= 1;
        }
      }
      c
    }).collect()
  }
}