Wildfire Sequel- Amazon Top Interview Questions
Problem Statement :
You are given a two-dimensional integer matrix matrix where
0 represents empty cell
1 represents a person
2 represents fire
3 represents a wall
You can assume there's only one person and in each turn fire expands in all four directions although fire can't expand through walls.
Return whether the person can move to either the top left corner or the bottom right corner. In each turn, the person moves first, then the fire expands. If the person makes it to either square as the same time as the fire, then they're safe.
Note that if you go a the square and then the fire expands in the same turn to the same square, you still survive.
Constraints
1 ≤ n, m < 250 where n and m are the number of rows and columns in matrix
Example 1
Input
matrix = [
[0, 0, 0],
[0, 1, 0],
[0, 2, 0]
]
Output
True
Explanation
The person can get to the top left corner.
Example 2
Input
matrix = [
[0, 2, 0],
[0, 1, 0],
[0, 2, 0]
]
Output
False
Explanation
There's fire getting in the way of the top left and bottom right corners.
Solution :
Solution in C++ :
bool solve(vector<vector<int>>& matrix) {
int a, b, x, y, e, n = matrix.size(), m = matrix[0].size();
queue<pair<pair<int, int>, int>> bfs;
vector<int> dir{1, 0, -1, 0, 1};
for (int i = 0; i < matrix.size(); i++) {
for (int j = 0; j < matrix[0].size(); j++) {
if (matrix[i][j] == 1) {
bfs.push({{i, j}, matrix[i][j]});
}
}
}
for (int i = 0; i < matrix.size(); i++) {
for (int j = 0; j < matrix[0].size(); j++) {
if (matrix[i][j] == 2) {
bfs.push({{i, j}, matrix[i][j]});
}
}
}
while (!bfs.empty()) {
int s = bfs.size();
for (int i = 0; i < s; i++) {
a = bfs.front().first.first;
b = bfs.front().first.second;
e = bfs.front().second;
if (a == 0 && b == 0 && e == 1) {
return true;
}
if (a == n - 1 && b == m - 1 && e == 1) {
return true;
}
for (int j = 0; j < 4; j++) {
x = a + dir[j];
y = b + dir[j + 1];
if (x < 0 || y < 0 || x == matrix.size() || y == matrix[0].size()) {
continue;
}
if (matrix[x][y] == 3 || matrix[x][y] == 2) {
continue;
}
if (e == 1) {
if (matrix[x][y] == -1) {
continue;
}
if (x == 0 && y == 0 && matrix[i][j] != 2) {
return true;
}
if (x == n - 1 && y == m - 1 && matrix[i][j] != 2) {
return true;
}
matrix[x][y] = -1;
bfs.push({{x, y}, 1});
}
if (e == 2) {
matrix[x][y] = 2;
bfs.push({{x, y}, 2});
}
}
bfs.pop();
}
}
return false;
}
Solution in Java :
import java.util.*;
class Solution {
static int N;
static int M;
static int[][] matrix;
static int[][] fire;
static int[][] escape;
public boolean solve(int[][] m) {
matrix = m;
N = matrix.length;
M = matrix[0].length;
fire = floodfill(2);
escape = floodfill(1);
return (works(0, 0) || works(N - 1, M - 1));
}
public static boolean works(int r, int c) {
return (escape[r][c] < Integer.MAX_VALUE && escape[r][c] <= fire[r][c]);
}
public static int[][] floodfill(int root) {
int[][] time = new int[N][M];
for (int i = 0; i < N; i++) Arrays.fill(time[i], Integer.MAX_VALUE);
ArrayDeque<int[]> bfs = new ArrayDeque<int[]>();
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
if (matrix[i][j] == root) {
time[i][j] = 0;
bfs.add(new int[] {i, j});
}
}
}
int[][] dirs = {{-1, 0}, {1, 0}, {0, 1}, {0, -1}};
while (!bfs.isEmpty()) {
int[] cell = bfs.pollFirst();
for (int[] dir : dirs) {
int newR = cell[0] + dir[0];
int newC = cell[1] + dir[1];
if (newR >= 0 && newC >= 0 && newR < N && newC < M && matrix[newR][newC] != 3
&& time[newR][newC] == Integer.MAX_VALUE) {
time[newR][newC] = time[cell[0]][cell[1]] + 1;
bfs.add(new int[] {newR, newC});
}
}
}
return time;
}
}
Solution in Python :
class Solution:
def solve(self, A):
INF = int(1e9)
R, C = len(A), len(A[0])
def neighbors(r, c):
for nr, nc in [[r - 1, c], [r, c - 1], [r + 1, c], [r, c + 1]]:
if 0 <= nr < R and 0 <= nc < C and A[nr][nc] != 3:
yield nr, nc
def bfs(queue) -> dict:
dist = {node: 0 for node in queue}
while queue:
node = queue.popleft()
for nei in neighbors(*node):
if nei not in dist:
dist[nei] = dist[node] + 1
queue.append(nei)
return dist
qfire = collections.deque()
qperson = collections.deque()
for r, row in enumerate(A):
for c, v in enumerate(row):
if v == 1:
qperson.append((r, c))
elif v == 2:
qfire.append((r, c))
dist_fire = bfs(qfire)
dist_person = bfs(qperson)
for place in ((0, 0), (R - 1, C - 1)):
if dist_fire.get(place, INF) >= dist_person.get(place, 2 * INF):
return True
return False
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