private Pair<Integer, Integer> findUncoveredZero() {
    for (int r = 0; r < rows; r++) {
      if (leastInRow[r] == 0) {
        return new Pair<>(r, leastInRowIndex[r]);
      }
    }
    return null;
  }
Find a zero cost assignment which is not covered. If there are no zero cost assignments which
are uncovered, then null will be returned.
@return pair of row and column indices of an uncovered zero or null
hbase-server/src/main/java/org/apache/hadoop/hbase/util/MunkresAssignment.java