Multiply String

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1.引入Array保存结果！之前想直接用string来操作，搞晕了也没搞出来。

2. 用StringBuilder

public class Solution {    public String multiply(String num1, String num2) {       if (num1 == null || num2 == null) {           return null;       }       int len1 = num1.length();       int len2 = num2.length();       int len3 = len1 + len2;              int[] num = new int[len3];       for (int i = len1 - 1; i >= 0; i--) {           int carry = 0;           for (int j = len2 - 1; j >= 0; j--) {               int product = num[i + j + 1] + carry + (num1.charAt(i) - '0')*(num2.charAt(j) - '0');               num[i + j + 1] = product % 10;               carry = product / 10;           }           num[i] = carry;       }              int i = 0;       while (i < len3-1 && num[i]== 0) {           i++;       }              StringBuilder sb = new StringBuilder();       while (i < len3) {           sb.append(num[i]);           i++;       }              return sb.toString();    }}

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