286. Walls and Gates

You are given a m x n 2D grid initialized with these three possible values.

1. `-1` - A wall or an obstacle.
2. `0` - A gate.
3. `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
``````

``````class Solution {
public:
void wallsAndGates(vector<vector<int>>& rooms) {
for (int i = 0; i < rooms.size(); ++i) {
for (int j = 0; j < rooms[i].size(); ++j) {
if (rooms[i][j] == 0) dfs(rooms, i, j, 0);
}
}
}
void dfs(vector<vector<int>>& rooms, int i, int j, int val) {
if (i < 0 || i >= rooms.size() || j < 0 || j >= rooms[i].size() || rooms[i][j] < val) return;
rooms[i][j] = val;
dfs(rooms, i + 1, j, val + 1);
dfs(rooms, i - 1, j, val + 1);
dfs(rooms, i, j + 1, val + 1);
dfs(rooms, i, j - 1, val + 1);
}
};
``````

``````class Solution {
public:
void wallsAndGates(vector<vector<int>>& rooms) {
queue<pair<int, int>> q;
vector<vector<int>> dirs{{0, -1}, {-1, 0}, {0, 1}, {1, 0}};
for (int i = 0; i < rooms.size(); ++i) {
for (int j = 0; j < rooms[i].size(); ++j) {
if (rooms[i][j] == 0) q.push({i, j});
}
}
while (!q.empty()) {
int i = q.front().first, j = q.front().second; q.pop();
for (int k = 0; k < dirs.size(); ++k) {
int x = i + dirs[k][0], y = j + dirs[k][1];
if (x < 0 || x >= rooms.size() || y < 0 || y >= rooms[0].size() || rooms[x][y] < rooms[i][j] + 1) continue;
rooms[x][y] = rooms[i][j] + 1;
q.push({x, y});
}
}
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/286

Surrounded Regions

Number of Islands

Shortest Distance from All Buildings

Robot Room Cleaner

Rotting Oranges

https://leetcode.com/problems/walls-and-gates/

https://leetcode.com/problems/walls-and-gates/discuss/72745/Java-BFS-Solution-O(mn)-Time

https://leetcode.com/problems/walls-and-gates/discuss/72746/My-short-java-solution-very-easy-to-understand

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