java: add «861. Score After Flipping Matrix»

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

java/score-after-flipping-matrix.java

@@ -0,0 +1,22 @@
+class Solution {
+  public int matrixScore(int[][] grid) {
+    int rows = grid.length;
+    int cols = grid[0].length;
+
+    int score = rows * (1 << (cols - 1));
+
+    for (int x = 1; x < cols; ++x) {
+      int same = 0;
+      for (int y = 0; y < rows; ++y) {
+        if (grid[y][x] == grid[y][0]) {
+          ++same;
+        }
+      }
+
+      same = Math.max(same, rows - same);
+      score += same * (1 << (cols - x - 1));
+    }
+
+    return score;
+  }
+}