[LeetCode]Multiply Strings

最新推荐文章于 2022-05-13 12:53:03 发布

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本文提供了两种使用C++实现字符串相乘的方法。第一种方法通过逐位相乘并累加的方式得出结果，注意进位处理及结果前导零的去除。第二种方法同样采用逆序处理输入字符串，并通过循环迭代计算乘积，最终返回正确格式的结果。

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class Solution {
//draw it out and then observe it to find out the regular pattern
//more practice is needed
public:
	string multiply(string num1, string num2) {
		// Start typing your C/C++ solution below
		// DO NOT write int main() function
		if(num1.size() == 0 || num2.size() == 0) return string("");
		string ans(num1.size()+num2.size()+1, '0');//important observation
		reverse(num1.begin(), num1.end());
		reverse(num2.begin(), num2.end());
		for (int digIdx2 = 0; digIdx2 < num2.size(); ++digIdx2)//the below number num2
		{
			int dig2 = num2[digIdx2]-'0';
			int carry = 0;
			for (int digIdx1 = 0; digIdx1 < num1.size(); ++digIdx1)//the up number num1 
			{
				int dig1 = num1[digIdx1]-'0';
				int tmp = ans[digIdx1+digIdx2]-'0';
				int sum_cur = carry+dig1*dig2+tmp;
				ans[digIdx1+digIdx2] = sum_cur%10+'0';
				carry = sum_cur/10;
			}
			ans[digIdx2+num1.size()] = carry+'0';	
		}
		reverse(ans.begin(), ans.end());

		int posStart = ans.find_first_not_of('0');
		if(posStart < 0 || posStart >= ans.size()) //in case of ""000"
			posStart = ans.size()-1;
		int len = ans.size()-posStart;
		return ans.substr(posStart, len);
	}
};

second time

class Solution {
public:
    string multiply(string num1, string num2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        int carry = 0;
        string result;
        for(int len = 0; ;++len)
        {
            int sum = 0;
            bool flag = false;
            for(int i = 0; i < num1.size(); ++i)
            {
                int j = len-i;
                if(j < 0 || j >= num2.size()) continue;
                sum += (num1[i]-'0')*(num2[j]-'0');
                flag = true;
            }
            if(!flag) break;
            sum += carry;
            result.push_back(sum%10+'0');
            carry = sum/10;
        }
        
        while(carry > 0)
        {
            result.push_back(carry%10+'0');
            carry /= 10;
        }
        reverse(result.begin(), result.end());
        if(result.find_first_not_of('0') == string::npos) return "0";
        else return result.substr(result.find_first_not_of('0'), result.size()-result.find_first_not_of('0'));
    }
};