Multiply Strings

Multiply Strings

问题：

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

思路：

　　手工模拟乘法

我的代码：

public class Solution {
    public String multiply(String num1, String num2) {
        if(num1==null || num1.length()==0 || num2==null || num2.length()==0 || num1.charAt(0)=='0' || num2.charAt(0)=='0')    return "0";
        if(num1.charAt(0) == '0') num1 = num1.substring(1);
        if(num2.charAt(0) == '0') num2 = num2.substring(1);
        int one = num1.length();
        int two = num2.length();
        String[] rst = new String[one];
        
        for(int i=one-1; i>=0; i--)
        {
            StringBuffer sb = new StringBuffer();
            for(int j=one-1; j>i; j--) sb.append('0');
            int a = (int)(num1.charAt(i)-'0');
            int carry = 0;
            for(int j=two-1; j>=0; j--)
            {
                int b = (int)(num2.charAt(j)-'0');
                int c = a*b + carry;
                sb.append(c%10);
                carry = c/10;
            }
            if(carry != 0)  sb.append(carry);
            rst[i] = sb.toString();
        }
        
        int maxLength = Integer.MIN_VALUE;
        for(String s: rst)  maxLength = Math.max(maxLength, s.length());
        StringBuffer sb = new StringBuffer();
        int carry = 0;
        for(int i=0; i<maxLength; i++)
        {
            int sum = carry;
            for(String s: rst)
            {
                sum += (s.length()<=i?0:(int)(s.charAt(i)-'0'));
            }
            sb.append(sum%10);
            carry = sum/10;
        }
        if(carry != 0) sb.append(carry);
        return sb.reverse().toString();
        
    }
    
}

他人代码：

public class Solution {
    public String multiply(String num1, String num2) {
        if (num1 == null || num2 == null) {
            return null;
        }
        
        int len1 = num1.length(), len2 = num2.length();
        int len3 = len1 + len2;
        int i, j, product, carry;

        int[] num3 = new int[len3];
        for (i = len1 - 1; i >= 0; i--) {
            carry = 0;
            for (j = len2 - 1; j >= 0; j--) {
                product = carry + num3[i + j + 1] +
                    Character.getNumericValue(num1.charAt(i)) *
                    Character.getNumericValue(num2.charAt(j));
                num3[i + j + 1] = product % 10;
                carry = product / 10;
            }
            num3[i + j + 1] = carry;
        }

        StringBuilder sb = new StringBuilder();
        i = 0;

        while (i < len3 - 1 && num3[i] == 0) {
            i++;
        }

        while (i < len3) {
            sb.append(num3[i++]);
        }

        return sb.toString();
    }
}

学习之处：

• 记住一个结论：两个数相乘，位数不会大于二者的位数之和，所以这样可以提前设定结果数组的大小了，不用再像我用了string的数组去存储中间结果，浪费空间，浪费时间
• 尽量少的用a,b等命名，用product多大气啊，另外能直接运算的就直接运算把，别再用变量进行记录传递了，关键是没作用，看起来代码啰嗦。