java: add «2812. Find the Safest Path in a Grid»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/find-the-safest-path-in-a-grid.java

@@ -0,0 +1,93 @@
+class Solution {
+  private final int[][] DIRECTIONS = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
+
+  private boolean inBounds(int[][] graph, int y, int x) {
+    int n = graph.length;
+    return y >= 0 && y < n && x >= 0 && x < n;
+  }
+
+  private void setSafeness(int[][] graph, Queue<int[]> queue) {
+    while (!queue.isEmpty()) {
+      int size = queue.size();
+
+      while (size-- > 0) {
+        int[] cell = queue.poll();
+        int y = cell[0], x = cell[1];
+
+        for (int[] d : DIRECTIONS) {
+          int ny = y + d[0];
+          int nx = x + d[1];
+
+          int value = graph[y][x];
+
+          if (inBounds(graph, ny, nx) && graph[ny][nx] == -1) {
+            graph[ny][nx] = value + 1;
+            queue.add(new int[] {ny, nx});
+          }
+        }
+      }
+    }
+  }
+
+  private int[][] toGraph(List<List<Integer>> grid) {
+    int n = grid.size();
+
+    int[][] graph = new int[n][n];
+
+    Queue<int[]> queue = new LinkedList<>();
+    for (int i = 0; i < n * n; ++i) {
+      int y = i / n;
+      int x = i % n;
+
+      if (grid.get(y).get(x) == 1) {
+        queue.add(new int[] {y, x});
+        graph[y][x] = 0;
+      } else {
+        graph[y][x] = -1;
+      }
+    }
+
+    // calculate safeness
+    setSafeness(graph, queue);
+
+    return graph;
+  }
+
+  private int safestPath(int[][] graph) {
+    int n = graph.length;
+
+    PriorityQueue<int[]> pq = new PriorityQueue<>((l, r) -> r[2] - l[2]);
+
+    pq.add(new int[] {0, 0, graph[0][0]});
+    graph[0][0] = -1;
+
+    while (!pq.isEmpty()) {
+      int[] next = pq.poll();
+
+      int y = next[0], x = next[1], safeness = next[2];
+
+      // found the goal
+      if (y == n - 1 && x == n - 1) {
+        return safeness;
+      }
+
+      for (int[] d : DIRECTIONS) {
+        int ny = y + d[0];
+        int nx = x + d[1];
+
+        if (inBounds(graph, ny, nx) && graph[ny][nx] != -1) {
+          pq.add(new int[] {ny, nx, Math.min(safeness, graph[ny][nx])});
+          graph[ny][nx] = -1;
+        }
+      }
+    }
+
+    return -1;
+  }
+
+  public int maximumSafenessFactor(List<List<Integer>> grid) {
+    int n = grid.size();
+    int[][] graph = toGraph(grid);
+    return safestPath(graph);
+  }
+}