[LeetCode] Walls and Gates 墙和门

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

231 - 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

这道题类似一种迷宫问题，规定了-1表示墙，0表示门，让求每个点到门的最近的曼哈顿距离，这其实类似于求距离场Distance Map的问题，那么我们先考虑用DFS来解，思路是，我们搜索0的位置，每找到一个0，以其周围四个相邻点为起点，开始DFS遍历，并带入深度值1，如果遇到的值大于当前深度值，我们将位置值赋为当前深度值，并对当前点的四个相邻点开始DFS遍历，注意此时深度值需要加1，这样遍历完成后，所有的位置就被正确地更新了，参见代码如下：

解法一：

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

上述的代码可以稍稍简化一下，我们将rooms[i][j]和val的大小判断放入起始判断条件中，这样我们就可以以门的位置开始调用DFS函数了，参见代码如下：

解法二：

// DFS
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);
}
};

那么下面我们再来看BFS的解法，却要借助queue，我们首先把门的位置都排入queue中，然后开始循环，对于门位置的四个相邻点，我们判断其是否在矩阵范围内，并且位置值是否大于上一位置的值加1，如果满足这些条件，我们将当前位置赋为上一位置加1，并将次位置排入queue中，这样等queue中的元素遍历完了，所有位置的值就被正确地更新了，参见代码如下：

解法三：

// BFS
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});
}
}
}
};

参考资料：

https://leetcode.com/discuss/78333/my-short-java-solution-very-easy-to-understand

https://leetcode.com/discuss/83543/17-lines-concise-and-easy-understand-c-solution

LeetCode All in One 题目讲解汇总(持续更新中...)