Note: Standard combination generation:

1. Do not return when find diff < 0. Because i + 1 could satisfy.

2. Since it does not return, it requires check whether it reaches end of needs.

class Solution {    public int shoppingOffers(List
     
       price, List
      
       
        > special, List
        
          needs) {        return generateComb(price, special, needs, new HashMap<>());    }        private int generateComb(List
         
           price, List
          
           
            > special, List
            
              needs, Map
             
              
               , Integer> map) { if (map.containsKey(needs)) return map.get(needs); int currentValue = getValue(price, needs); for (List
               
                 s : special) { List
                
                  clone = new ArrayList<>(needs); int i = 0; for (; i < clone.size(); i++) { int diff = clone.get(i) - s.get(i); if (diff < 0) break; clone.set(i, diff); } if (i == needs.size()) currentValue = Math.min(currentValue, s.get(s.size() - 1) + generateComb(price, special, clone, map)); } map.put(needs, currentValue); return currentValue; } private int getValue(List
                 
                   price, List
                  
                    needs) { int result = 0; for (int i = 0; i < price.size(); i++) { result += price.get(i) * needs.get(i); } return result; }}