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

 public class Solution {
     public String multiply(String num1, String num2) {
         if (num1 == null || num2 == null) {
             return null;
         }

         int len1 = num1.length();
         int len2 = num2.length();

         int[] product = new int[len1 + len2];

         // 计算相应位置的product.
         for (int i = 0; i < len1; i++) {
             for (int j = 0; j < len2; j++) {
                 // 注意，这里要使用+=以不断累加乘积
                 product[i + j] += (num1.charAt(len1 - 1 - i) - '0') * (num2.charAt(len2 - 1 - j) - '0');
             }
         }

         StringBuilder ret = new StringBuilder();

         int carry = 0;
         // 计算进位
         for (int i = 0; i < len1 + len2; i++) {
             product[i] = product[i] + carry;
             int digit = product[i] % 10;
             carry = product[i] / 10;
             ret.insert(0, digit);
         }

         // 去掉前导0
         while (ret.length() > 1 && ret.charAt(0) == '0') {
             ret.deleteCharAt(0);
         }

         return ret.toString();
     }
 }

2015.1.20 redo

 public String multiply(String num1, String num2) {
         // 18:24
         if (num1 == null || num2 == null) {
             return null;
         }

         int len1 = num1.length();
         int len2 = num2.length();
         int[] sum = new int[len1 + len2];

         // sum all of the mutiply result.
         for (int i = ; i < len1; i++) {
             for (int j = ; j < len2; j++) {
                 sum[i + j] += (num1.charAt(len1 -  - i) - '') * (num2.charAt(len2 -  - j) - '');
             }
         }

         int carry = ;
         for (int i = ; i < len1 + len2; i++) {
             sum[i] = sum[i] + carry;

             // Bug1: this line should be processed first.
             carry = sum[i] / ;
             sum[i] %= ;
         }

         StringBuilder sb = new StringBuilder();
         for (int i = ; i < len1 + len2; i++) {
             sb.insert(, sum[i] + "");
         }

         // delete the leading "0"
         while (sb.charAt() == '' && sb.length() != ) {
             sb.deleteCharAt();
         }

         return sb.toString();
     }

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