LeetCode: Multiply Strings 解题报告

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.

SOLUTION 1:

参考自http://blog.csdn.net/fightforyourdream/article/details/17370495

相当优雅的算法，主页君稍微改进，把翻转String这一步拿掉了。比起一般的做法，这个算法很容易就BUG FREE.

思路：
1 建立数组，双层循环遍历两个string，把单位的乘积累加到数组相应的位置
2 处理进位并输出
3 注意前导零的corner case

 1 public class Solution {
 2     public String multiply(String num1, String num2) {
 3         if (num1 == null || num2 == null) {
 4             return null;
 5         }
 6
 7         int len1 = num1.length();
 8         int len2 = num2.length();
 9
10         int[] product = new int[len1 + len2];
11
12         // 计算相应位置的product.
13         for (int i = 0; i < len1; i++) {
14             for (int j = 0; j < len2; j++) {
15                 // 注意，这里要使用+=以不断累加乘积
16                 product[i + j] += (num1.charAt(len1 - 1 - i) - ‘0‘) * (num2.charAt(len2 - 1 - j) - ‘0‘);
17             }
18         }
19
20         StringBuilder ret = new StringBuilder();
21
22         int carry = 0;
23         // 计算进位
24         for (int i = 0; i < len1 + len2; i++) {
25             product[i] = product[i] + carry;
26             int digit = product[i] % 10;
27             carry = product[i] / 10;
28             ret.insert(0, digit);
29         }
30
31         // 去掉前导0
32         while (ret.length() > 1 && ret.charAt(0) == ‘0‘) {
33             ret.deleteCharAt(0);
34         }
35
36         return ret.toString();
37     }
38 }

请至主页群GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/string/Multiply.java