学步园

字符串的相乘，可用于解决大数相乘，注意首尾颠倒

class Solution {
public:
string multiply(string num1, string num2) {
      if(num1 == "0" || num2 == "0") return "0";
      int l1 = num1.length(), l2 = num2.length();
      int n1[l1+1], n2[l2+1], ans[l1+l2+1];
      memset(ans, 0, sizeof(ans));
      for(int i = 0; i < l1; ++i)
        n1[i] = num1[i] - '0';
      for(int i = 0; i < l2; ++i)
        n2[i] = num2[i] - '0';

      for(int i = 0; i < l1; ++i)
        for(int j = 0; j < l2; ++j)
          ans[i+j+1] += n1[i]*n2[j];
      string ss = "";
      for(int k = l1+l2-1; k >= 0; --k) {
        if(k > 0) ans[k-1] += ans[k]/10;
        ans[k] %= 10;
        ss = char(ans[k]+'0')+ss;
      }
      ss = ss[0] == '0' ? ss.substr(1):ss;
      return ss;
    }
};