getMulliganIndices() {
    const mulliganIndices = [];
    if (this.getIsChooseHand()) {
      const cardNodes = this.getBottomDeckLayer().getCardNodes();

      // get mulligan indices from selected cards
      for (let i = 0, il = cardNodes.length; i < il; i++) {
        const cardNode = cardNodes[i];
        const cardIndex = cardNode.getHandIndex();
        if (cardIndex != null && cardNode.getSelected()) {
          mulliganIndices.push(cardIndex);
        }
      }
    }

    return mulliganIndices;
  }
Returns an array of indices chosen by the player to be mulliganed.
@returns {Array}