add: MatrixInfection

Author: Jer3myYu

java/Graphs/MatrixInfection.java

@@ -0,0 +1,53 @@
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+public class MatrixInfection {
+    public int matrixInfection(int[][] matrix) {
+        int[][] dirs = new int[][]{{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
+        Deque<int[]> queue = new ArrayDeque<>();
+        int ones, seconds;
+        ones = seconds = 0;
+        // Count the total number of uninfected cells and add each infected 
+        // cell to the queue to represent level 0 of the level-order  
+        // traversal.
+        for (int r = 0; r < matrix.length; r++) {
+            for (int c = 0; c < matrix[0].length; c++) {
+                if (matrix[r][c] == 1) {
+                    ones += 1;
+                } else if (matrix[r][c] == 2) {
+                    queue.offer(new int[]{r, c});
+                }
+            }
+        }
+        // Use level-order traversal to determine how long it takes to 
+        // infect the uninfected cells.
+        while (!queue.isEmpty() && ones > 0) {
+            // 1 second passes with each level of the matrix that's explored.
+            seconds++;
+            int size = queue.size();
+            for (int i = 0; i < size; i++) {
+                int[] pos = queue.poll();
+                int r = pos[0];
+                int c = pos[1];
+                // Infect any neighboring 1s and add them to the queue to be 
+                // processed in the next level.
+                for (int[] d : dirs) {
+                    int nextR = r + d[0];
+                    int nextC = c + d[1];
+                    if (isWithinBounds(nextR, nextC, matrix) && matrix[nextR][nextC] == 1) {
+                        matrix[nextR][nextC] = 2;
+                        ones--;
+                        queue.offer(new int[]{nextR, nextC});
+                    }
+                }
+            }
+        }
+        // If there are still uninfected cells left, return -1. Otherwise, 
+        // return the time passed.
+        return ones == 0 ? seconds : -1;
+    }
+
+    private boolean isWithinBounds(int r, int c, int[][] matrix) {
+        return 0 <= r && r < matrix.length && 0 <= c && c < matrix[0].length;
+    }
+}