private Pair<Integer, Integer> primeInRow(int r) {
    for (int c = 0; c < cols; c++) {
      if (mask[r][c] == PRIME) {
        return new Pair<>(r, c);
      }
    }
    return null;
  }
Find a primed zero in the specified row. If there are no primed zeroes in the specified row,
then null will be returned.
@param r the index of the row to be searched
@return pair of row and column indices of primed zero or null
hbase-server/src/main/java/org/apache/hadoop/hbase/util/MunkresAssignment.java