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.

https://oj.leetcode.com/problems/multiply-strings/

思路：大数乘法，模拟笔算过程即可。

注意的细节：

1. 前导0去掉；
2. 结果为0；

代码：

import java.util.Arrays;public class Solution {	public String multiply(String num1, String num2) {		if (num1 == null || num2 == null || num1.length() == 0				|| num2.length() == 0)			return null;		int n1 = num1.length();		int n2 = num2.length();		char[] str1 = num1.toCharArray();		char[] str2 = num2.toCharArray();		char[] prod = new char[n1 + n2];		Arrays.fill(prod, ‘0‘);		int i, j;		for (i = 0; i < n1; i++)			for (j = 0; j < n2; j++) {				prod[i + j] = (char) ((prod[i + j] - ‘0‘)						+ (str1[n1 - 1 - i] - ‘0‘) * (str2[n2 - 1 - j] - ‘0‘) + ‘0‘);			}		int c = 0;		for (i = 0; i < n1 + n2; i++) {			prod[i] = (char) ((prod[i] - ‘0‘) + c + ‘0‘);			if (prod[i] - ‘0‘ + c > 9) {				c = (prod[i] - ‘0‘) / 10;				prod[i] = (char) ((prod[i] - ‘0‘) % 10 + ‘0‘);			} else				c = 0;		}		StringBuilder sb = new StringBuilder();		i = n1 + n2 - 1;		while (i >= 0 && prod[i] == ‘0‘)			i--;		if (i == -1) {			sb.append("0");			return sb.toString();		}		for (; i >= 0; i--) {			sb.append(prod[i]);		}		return sb.toString();	}	public static void main(String[] args) {		System.out.println(new Solution().multiply("123", "12345"));		System.out.println(new Solution().multiply("0", "123"));			}}

参考：

http://www.cnblogs.com/TenosDoIt/p/3735309.html

http://leetcodenotes.wordpress.com/2013/10/20/leetcode-multiply-strings-%E5%A4%A7%E6%95%B4%E6%95%B0%E7%9A%84%E5%AD%97%E7%AC%A6%E4%B8%B2%E4%B9%98%E6%B3%95/comment-page-1/#comment-122