【重点】【BFS】994.腐烂的橘子

题目
非常类似的题目：542.01矩阵

法1：BFS

深刻理解！此方法类似二叉树层次搜索！！！

Python

class Solution:
    def orangesRotting(self, grid: List[List[int]]) -> int:
        m, n = len(grid), len(grid[0])
        fresh = 0
        queue = collections.deque()
        for i in range(m):
            for j in range(n):
                if grid[i][j] == 1:
                    fresh += 1
                elif grid[i][j] == 2:
                    queue.append([i, j])
        res = 0
        dx = [1, -1, 0, 0]
        dy = [0, 0, 1, -1]
        while queue and fresh > 0:
            tmp_size = len(queue)
            res += 1
            for _ in range(tmp_size):
                [x, y] = queue.popleft()
                for i in range(4):
                    newX = x + dx[i]
                    newY = y + dy[i]
                    if (0 <= newX < m and 0 <= newY < n and grid[newX][newY] == 1):
                        grid[newX][newY] = 2
                        fresh -= 1
                        queue.append([newX, newY])
        
        return res if fresh == 0 else -1

Java

此方法类似二叉树层次搜索。

class Solution {
    public int orangesRotting(int[][] grid) {
        int[] dx = {-1, 1, 0, 0}; // 方向数组
        int[] dy = {0, 0, -1, 1};
        Queue<int[]> queue = new LinkedList<>();
        int m = grid.length, n = grid[0].length, res = 0, flash = 0;
        int[][] copyGrid = new int[m][n]; // 复制，保证不改变原始输入数组
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                copyGrid[i][j] = grid[i][j];
                if (grid[i][j] == 1) {
                    ++flash;
                } else if (grid[i][j] == 2) {
                    queue.offer(new int[]{i, j});
                }
            }
        }
        
        while (flash > 0 && !queue.isEmpty()) {
            ++res;
            int tmpSize = queue.size();
            for (int i = 0; i < tmpSize; ++i) {
                int[] loc = queue.poll();
                int x = loc[0], y = loc[1];
                for (int k = 0; k < 4; ++k) {
                    int newX = x + dx[k];
                    int newY = y + dy[k];
                    if ((newX >= 0 && newX < m) 
                            && (newY >= 0 && newY < n)
                            && copyGrid[newX][newY] == 1) {
                        copyGrid[newX][newY] = 2;
                        queue.offer(new int[]{newX, newY});
                        --flash;
                    }
                }
            }
        }

        if (flash > 0) {
            return -1;
        } 

        return res;
    }
}

版本2，code

class Solution {
    public int orangesRotting(int[][] grid) {
        int ans = 0, flash = 0, m = grid.length, n = grid[0].length;
        Queue<int[]> queue = new LinkedList<>();
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid[i][j] == 1) {
                    ++flash;
                } else if (grid[i][j] == 2) {
                    queue.offer(new int[]{i, j});
                }
            }
        }

        while (flash > 0 && !queue.isEmpty()) {
            ++ans;
            int size = queue.size();
            for (int i = 0; i < size; ++i) {
                int[] loc = queue.poll();
                int x = loc[0], y = loc[1];
                if (x - 1 >= 0) {
                    if (grid[x - 1][y] == 1) {
                        grid[x - 1][y] = 2;
                        --flash;
                        queue.offer(new int[]{x - 1, y});
                    }
                }
                if (x + 1 < m) {
                    if (grid[x + 1][y] == 1) {
                        grid[x + 1][y] = 2;
                        --flash;
                        queue.offer(new int[]{x + 1, y});
                    }
                }
                if (y - 1 >= 0) {
                    if (grid[x][y - 1] == 1) {
                        grid[x][y - 1] = 2;
                        --flash;
                        queue.offer(new int[]{x, y - 1});
                    }
                }
                if (y + 1 < n) {
                    if (grid[x][y + 1] == 1) {
                        grid[x][y + 1] = 2;
                        --flash;
                        queue.offer(new int[]{x, y + 1});
                    }
                }
            }
        }
        return flash > 0 ? -1 : ans;
    }
}

542. 01 矩阵

题目

class Solution:
    def updateMatrix(self, mat: List[List[int]]) -> List[List[int]]:
        m, n = len(mat), len(mat[0])
        res = [[0] * n for _ in range(m)]
        q = collections.deque()
        dis = 0

        for i in range(m):
            for j in range(n):
                if mat[i][j] == 0:
                    res[i][j] = 0
                    q.append([i, j])
        
        while q:
            tmpSize = len(q)
            dis += 1
            for i in range(tmpSize):
                [x, y] = q.popleft()
                dx = [1, -1, 0, 0]
                dy = [0, 0, 1, -1]
                for i in range(4):
                    newX = x + dx[i]
                    newY = y + dy[i]
                    if 0 <= newX < m and 0 <= newY < n and mat[newX][newY] == 1:
                        res[newX][newY] = dis
                        mat[newX][newY] = 0
                        q.append([newX, newY])
        
        return res