题解
Medium
BFS
这道题有点类似Number of Islands,可以用DFS或者BFS,但我觉得要更难一点,因为涉及到更新记忆数组。下面我的做法是用BFS(今天主要是想要练习BFS的套路)。
看了官方给出的答案,我的做法还是有几点值得改进的,其中一点是避免使用了记忆数组。因为从根本上来说,BFS是能够保证最短路径(这里是最短时间)的,一般用DFS的时候才需要配合使用记忆数组。我这里显然是没有在第一步把所有等于2的位置都放进队列中。可能是受Number of Islands那道题的影响。
class Solution {
public:
int orangesRotting(vector<vector<int>>& grid) {
if(grid.empty()) return 0;
int min_minutes = 0;
vector<vector<int>> memo(grid.size(), vector<int>(grid[0].size(), INT_MAX));
int dirs[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
for(int i = 0; i < grid.size(); i++) {
for(int j = 0; j < grid[0].size(); j++) {
if(grid[i][j] == 2) {
vector<vector<int>> temp = grid;
int minutes = 0;
queue<vector<int>> q;
q.push({i, j});
while(!q.empty()) {
int sz = q.size();
minutes++;
for(int i = 0; i < sz; i++) {
auto t = q.front();
q.pop();
for(int k = 0; k < 4; k++) {
int x = t[0] + dirs[k][0];
int y = t[1] + dirs[k][1];
if(x < 0 || x >= temp.size() || y < 0 || y >= temp[0].size()
|| temp[x][y] == 0)
continue;
if(temp[x][y] == 1) {
temp[x][y] = 2;
memo[x][y] = min(memo[x][y], minutes);
q.push({x, y});
}
}
}
}
}
}
}
for(int i = 0; i < grid.size(); i++) {
for(int j = 0; j < grid[0].size(); j++) {
if(grid[i][j] == 1) {
if(memo[i][j] == INT_MAX) return -1;
min_minutes = max(min_minutes, memo[i][j]);
}
}
}
return min_minutes;
}
};
下面根据官方答案,改进了上述代码。
class Solution {
public:
int orangesRotting(vector<vector<int>>& grid) {
if(grid.empty()) return 0;
int minutes = -1, num_of_fresh = 0;
queue<vector<int>> q;
for(int i = 0; i < grid.size(); i++) {
for(int j = 0; j < grid[0].size(); j++) {
if(grid[i][j] == 2) {
q.push({i, j});
} else if (grid[i][j] == 1) {
num_of_fresh++;
}
}
}
if(num_of_fresh == 0) return 0;
int dirs[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
while(!q.empty()) {
int sz = q.size();
minutes++;
for(int i = 0; i < sz; i++) {
auto t = q.front();
q.pop();
for(int k = 0; k < 4; k++) {
int x = t[0] + dirs[k][0];
int y = t[1] + dirs[k][1];
if(x < 0 || x >= grid.size() || y < 0 || y >= grid[0].size()
|| grid[x][y] == 0)
continue;
if(grid[x][y] == 1) {
grid[x][y] = 2;
num_of_fresh--;
q.push({x, y});
}
}
}
}
return num_of_fresh == 0 ? minutes : -1;
}
};