大整数相乘Multiply Strings

        
        public String multiply(String num1, String num2) {
            int[] num1IntArr = toIntArr(num1);
            int[] num2IntArr = toIntArr(num2);
            if (num1IntArr.length == 0 || num2IntArr.length == 0){
                return "";
            }
            
            if ("0".endsWith(num1.trim()) || "0".endsWith(num2.trim())){
                return "0";
            }
            
            
            Map<Integer, Integer> sumMap = new TreeMap<Integer, Integer>();
            for (int i = 0; i < num1IntArr.length; i++) {
                for (int j = 0; j < num2IntArr.length; j++) {
                    int a = num1IntArr[i];
                    int b = num2IntArr[j];
                    int sum = a*b;
                    if (sumMap.containsKey(i+j)){
                        int initValue = sumMap.get(i+j);
                        sumMap.put(i+j, initValue+sum);
                    }else {
                        sumMap.put(i+j, sum);
                    }
                }
            }
            
            for (Entry<Integer, Integer> entry: sumMap.entrySet()) {
                if (entry.getValue() > 9){
                    int preKey = entry.getKey() + 1;
                    if (sumMap.containsKey(preKey)){
                        int preValue = sumMap.get(preKey);
                        sumMap.put(preKey, preValue + entry.getValue() / 10);
                    }else {
                        sumMap.put(preKey, entry.getValue() / 10);
                    }
                    sumMap.put(entry.getKey(), entry.getValue() % 10);
                }
            }
            
            StringBuffer buffer = new StringBuffer();
            for (Entry<Integer, Integer> entry: sumMap.entrySet()) {
                buffer.append(entry.getValue());
            }
            
            return buffer.reverse().toString();
        }
        
        public int[] toIntArr(String num){
            int size = 0;
            String[] numStrArr = num.split("");
            for (int i = 0; i < numStrArr.length; i++) {
                String decStr = numStrArr[i];
                if (decStr.length() != 1){
                    continue;
                }
                size++;
            }
            
            int index = 0;
            int[] intArray = new int[size];
            for (int i = 0; i < numStrArr.length; i++) {
                String decStr = numStrArr[i];
                if (decStr.length() != 1){
                    continue;
                }
                intArray[size - 1 - index++] = Integer.parseInt(decStr);
            }
            
            return intArray;
        }