43 - Multiply Strings

 JAVA

• class Solution {
      public String multiply(String num1, String num2) {
          if ("0".equals(num1) || "0".equals(num2)) {
              return "0";
          }
          int m = num1.length(), n = num2.length();
          int[] arr = new int[m + n];
          for (int i = m - 1; i >= 0; --i) {
              int a = num1.charAt(i) - '0';
              for (int j = n - 1; j >= 0; --j) {
                  int b = num2.charAt(j) - '0';
                  arr[i + j + 1] += a * b;
              }
          }
          for (int i = arr.length - 1; i > 0; --i) {
              arr[i - 1] += arr[i] / 10;
              arr[i] %= 10;
          }
          int i = arr[0] == 0 ? 1 : 0;
          StringBuilder ans = new StringBuilder();
          for (; i < arr.length; ++i) {
              ans.append(arr[i]);
          }
          return ans.toString();
      }
  }

C++

• class Solution {
  public:
      string multiply(string num1, string num2) {
          if (num1 == "0" || num2 == "0") {
              return "0";
          }
          int m = num1.size(), n = num2.size();
          vector<int> arr(m + n);
          for (int i = m - 1; i >= 0; --i) {
              int a = num1[i] - '0';
              for (int j = n - 1; j >= 0; --j) {
                  int b = num2[j] - '0';
                  arr[i + j + 1] += a * b;
              }
          }
          for (int i = arr.size() - 1; i; --i) {
              arr[i - 1] += arr[i] / 10;
              arr[i] %= 10;
          }
          int i = arr[0] ? 0 : 1;
          string ans;
          for (; i < arr.size(); ++i) {
              ans += '0' + arr[i];
          }
          return ans;
      }
  };