Multiply Strings

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.

public class Solution {
    public String multiply(String num1, String num2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        String large = null;
        String small = null;
        if(num1.length() > num2.length()){
            large = num1;
            small = num2;
        }else{
            large = num2;
            small = num1;
        }
        if(num1.equals("0") || num2.equals("0")){
            return "0";
        }
        StringBuffer result = new StringBuffer();
        int carry = 0;
        for(int l = large.length() + small.length() - 2; l > -1; l --){
            int sum = carry;
            for(int j = small.length() - 1; j > -1; j --){
                int i = l - j;
                if(i < large.length() && i > -1){
                    sum += (large.charAt(i) - '0') * (small.charAt(j) - '0');
                }
            }
            carry = sum / 10;
            result.insert(0,sum % 10);
        }
        if(carry != 0){
            result.insert(0,carry);
        }
        return result.toString();
    }   
}

 第二遍:

利用array 可以简化运算！

public class Solution {
    public String multiply(String num1, String num2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if ( num1.equals("0") || num2.equals("0") ) return "0";
        int n1=num1.length(), n2 = num2.length();
        int[] c = new int[n1 + n2];
        for (int i = n1 - 1; i > -1; i--){
            for (int j = n2 - 1; j > -1; j--){
                c[i + j + 1] += (num1.charAt(i)-'0')*(num2.charAt(j)-'0');
            }
        }
        for (int o=n1 + n2 - 1; o > 0; o --){
            c[o - 1] += c[o]/10;
            c[o] %= 10;
        }
        int bit = 0;
        if(c[0] == 0) bit = 1;
        StringBuffer ret = new StringBuffer();
        for (int i=bit; i < n1 + n2; i ++)
            ret.append(c[i]);
        return ret.toString();
    }   
}

 第三遍：

public class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0") || num2.equals("0")) return "0";
        int len1 = num1.length(), len2 = num2.length();
        int[] result = new int[len1 + len2 - 1];
        for(int i = 0; i < len1; i ++){
            for(int j = 0; j < len2; j ++){
                result[i + j] += (num1.charAt(len1 - 1 - i) - '0') * (num2.charAt(len2 - 1 - j) - '0');
            }
        }
        int upp = 0;
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < result.length; i ++){
            int sum = result[i] + upp;
            sb.insert(0, sum % 10);
            upp = sum / 10;
        }
        if(upp != 0) sb.insert(0, upp);
        return sb.toString();
    }
}