class Solution {
public:
vector<vector<int>> res;
int m, n;
vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
m = matrix.size(); if (m == 0) return res;
n = matrix[0].size(); if (n==0) return res;
res = vector<vector<int>>(m, vector<int>(n, INT_MAX));
queue<pair<int,int>> q;
static int dirs[] = { -1, 0, 1, 0, -1 };
for (int i = 0; i < m; i++)
for (int j = 0; j < n; j++)
if (matrix[i][j] == 0) {
res[i][j] = 0;
q.push({i,j});
}
while (!q.empty()) {
auto p = q.front();
q.pop();
for (int k = 0; k < 4; k++) {
int x = p.first + dirs[k];
int y = p.second + dirs[k+1];
if (x < 0 || y < 0 || x >= m || y >= n || res[p.first][p.second] + 1 >= res[x][y]) continue;
res[x][y] = res[p.first][p.second] + 1;
q.push({x, y});
}
}
return res;
}
};
/* DFS: slow
class Solution {
public:
vector<vector<int>> res;
int m, n;
vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
m = matrix.size(); if (m == 0) return res;
n = matrix[0].size(); if (n==0) return res;
res = vector<vector<int>>(m, vector<int>(n, INT_MAX));
for (int i = 0; i < m; i++)
for (int j = 0; j < n; j++)
if (matrix[i][j] == 0) {
res[i][j] = 0;
dfs(matrix, i, j);
}
return res;
}
void dfs(vector<vector<int>>& mx, int i, int j) {
static int dirs[] = { -1, 0, 1, 0, -1 };
for (int d = 0; d < 4; d++)
{
int x = i + dirs[d];
int y = j + dirs[d+1];
if (x < 0 || x >= m || y < 0 || y >= n || res[i][j] + 1 >= res[x][y]) continue;
res[x][y] = res[i][j] + 1;
dfs(mx, x, y);
}
}
};
*/
/* each point BFS: too slow
class Solution {
public:
vector<vector<int>> res;
int m, n;
vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
m = matrix.size(); if (m == 0) return res;
n = matrix[0].size(); if (n==0) return res;
res = vector<vector<int>>(m, vector<int>(n, INT_MAX));
for (int i = 0; i < m; i++)
for (int j = 0; j < n; j++)
helper(matrix, i, j);
return res;
}
void helper(vector<vector<int>>& mx, int i, int j) {
static int dirs[] = { -1, 0, 1, 0, -1 };
if (mx[i][j] == 0) {
res[i][j] = 0;
return;
}
vector<vector<bool>> v(m, vector<bool>(n, false));
queue<pair<int,int>> q;
q.push({i,j});
v[i][j] = true;
int lv = 0;
while (!q.empty()) {
int qs = q.size();
lv++;
while (qs-- > 0) {
auto p = q.front();
q.pop();
for (int k = 0; k < 4; k++) {
int x = p.first + dirs[k];
int y = p.second + dirs[k+1];
if (x < 0 || y < 0 || x >= m || y >= n || v[x][y]) continue;
if (mx[x][y] == 0) {
res[i][j] = lv;
return;
}
else {
q.push({x, y});
v[x][y] = true;
}
}
}
}
}
};
*/
