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Question
Formatted question description: https://leetcode.ca/all/286.html
286 Walls and Gates
You are given a m x n 2D grid initialized with these three possible values.
-1 - A wall or an obstacle.
0 - A gate.
INF - Infinity means an empty room.
We use the value 2^31 - 1 = 2147483647 to represent INF as you may assume that
the distance to a gate is less than 2147483647.
Fill each empty room with the distance to its nearest gate.
If it is impossible to reach a gate, it should be filled with INF.
For example, given the 2D grid:
INF -1 0 INF
INF INF INF -1
INF -1 INF -1
0 -1 INF INF
After running your function, the 2D grid should be:
3 -1 0 1
2 2 1 -1
1 -1 2 -1
0 -1 3 4
@similar: 317 Shortest Distance from All Buildings
Algorithm
Search for the position of 0, and every time a 0 is found, start DFS traversal
with four adjacent points around it as the starting point, and bring in the depth value 1.
If the value encountered is greater than the current depth value, assign the position value to the current depth value, and start DFS traversal for the four adjacent points of the current point. Note that the depth value needs to be increased by 1.
After the traversal is completed, all positions are updated correctly
Code
Java
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public class Solution_dfs { public void wallsAndGates(int[][] rooms) { if (rooms == null || rooms.length == 0 || rooms[0].length == 0) { return; } int m = rooms.length; int n = rooms[0].length; for (int i = 0; i < m; i++) { for (int j = 0; j < n; j++) { if (rooms[i][j] == 0) { // start from every gate boolean[][] isVisited = new boolean[m][n]; // each new gate, associated with a new isVisited map fill(rooms, i, j, 0, isVisited); } } } } public void fill(int[][] rooms, int i, int j, int distance, boolean[][] isVisited) { int m = rooms.length; int n = rooms[0].length; // rooms[i][j] <= 0 meaning == -1 or == 0, both stop if (i < 0 || i >= m || j < 0 || j >= n || rooms[i][j] <= 0 || isVisited[i][j]) { return; } rooms[i][j] = Math.min(rooms[i][j], distance); isVisited[i][j] = true; fill(rooms, i - 1, j, distance + 1, isVisited); fill(rooms, i, j + 1, distance + 1, isVisited); fill(rooms, i + 1, j, distance + 1, isVisited); fill(rooms, i, j - 1, distance + 1, isVisited); isVisited[i][j] = false; } } public class Solution_bfs { public void wallsAndGates(int[][] rooms) { if (rooms == null || rooms.length == 0) return; int[][] dir = new int[][]{ {0, 1}, {0, -1}, {1, 0}, {-1, 0} }; Queue<Pair> queue = new LinkedList<>(); for (int i = 0; i < rooms.length; i++) { for (int j = 0; j < rooms[0].length; j++) { if (rooms[i][j] == 0) { queue.offer(new Pair(i, j)); } } } while (!queue.isEmpty()) { Pair p = queue.poll(); for (int[] d : dir) { int x = p.x + d[0]; int y = p.y + d[1]; if (x < 0 || x >= rooms.length || y < 0 || y >= rooms[0].length || rooms[x][y] <= rooms[p.x][p.y] + 1) // min check is here in if continue; rooms[x][y] = rooms[p.x][p.y] + 1; queue.offer(new Pair(x, y)); } } } }
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// OJ: https://leetcode.com/problems/walls-and-gates/ // Time: O(MN) // Space: O(MN) class Solution { public: void wallsAndGates(vector<vector<int>>& A) { int M = A.size(), N = A[0].size(), dirs[4][2] = { {0,1},{0,-1},{1,0},{-1,0} }; queue<pair<int, int>> q; for (int i = 0; i < M; ++i) { for (int j = 0; j < N; ++j) { if (A[i][j] == 0) q.emplace(i, j); } } while (q.size()) { int cnt = q.size(); while (cnt--) { auto [x, y] = q.front(); q.pop(); for (auto &[dx, dy] : dirs) { int a = x + dx, b = y + dy; if (a < 0 || b < 0 || a >= M || b >= N || A[a][b] <= A[x][y] + 1) continue; A[a][b] = A[x][y] + 1; q.emplace(a, b); } } } } };
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class Solution: def wallsAndGates(self, rooms: List[List[int]]) -> None: """ Do not return anything, modify rooms in-place instead. """ m, n = len(rooms), len(rooms[0]) inf = 2**31 - 1 q = deque([(i, j) for i in range(m) for j in range(n) if rooms[i][j] == 0]) d = 0 while q: d += 1 for _ in range(len(q)): i, j = q.popleft() for a, b in [[0, 1], [0, -1], [1, 0], [-1, 0]]: x, y = i + a, j + b if 0 <= x < m and 0 <= y < n and rooms[x][y] == inf: rooms[x][y] = d q.append((x, y)) ############ from collections import deque INF = 2147483647 class Solution(object): def wallsAndGates(self, rooms): """ :type rooms: List[List[int]] :rtype: void Do not return anything, modify rooms in-place instead. """ queue = deque([]) directions = [(1, 0), (-1, 0), (0, 1), (0, -1)] for i in range(0, len(rooms)): for j in range(0, len(rooms[0])): if rooms[i][j] == 0: queue.append((i, j)) while queue: i, j = queue.popleft() for di, dj in directions: p, q = i + di, j + dj if 0 <= p < len(rooms) and 0 <= q < len(rooms[0]) and rooms[p][q] == INF: rooms[p][q] = rooms[i][j] + 1 queue.append((p, q))