31-Rotten-Oranges.md

Rotten Oranges

Problem Link

https://leetcode.com/problems/rotting-oranges/

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Pattern

• Queue
• BFS

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Approach

Run multi-source BFS from all rotten oranges and spread the rot level by level.

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Time Complexity

O(m * n)

Space Complexity

O(m * n)

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Java Solution

import java.util.*;
class Solution {
    public int orangesRotting(int[][] grid) {
        int rows = grid.length, cols = grid[0].length, fresh = 0, minutes = 0;
        Queue<int[]> queue = new LinkedList<>();
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (grid[i][j] == 2) queue.offer(new int[]{i, j});
                if (grid[i][j] == 1) fresh++;
            }
        }
        int[][] dirs = {{1,0},{-1,0},{0,1},{0,-1}};
        while (!queue.isEmpty() && fresh > 0) {
            int size = queue.size();
            minutes++;
            for (int s = 0; s < size; s++) {
                int[] curr = queue.poll();
                for (int[] d : dirs) {
                    int r = curr[0] + d[0], c = curr[1] + d[1];
                    if (r >= 0 && c >= 0 && r < rows && c < cols && grid[r][c] == 1) {
                        grid[r][c] = 2;
                        fresh--;
                        queue.offer(new int[]{r, c});
                    }
                }
            }
        }
        return fresh == 0 ? minutes : -1;
    }
}