LeetCode 43 Multiply Strings（字符串相乘）

题目链接： https://leetcode.com/problems/multiply-strings/?tab=Description

求解大数相乘问题

按照上图所示，进行嵌套循环计算。每次计算出两个位置上的数字，并且对其求和进行更新操作。

下面这个讲解很赞！！！

https://discuss.leetcode.com/topic/30508/easiest-java-solution-with-graph-explanation/3 

参考代码： 

package leetcode_50;

/***
 *
 * @author pengfei_zheng
 * 实现大数相乘
 */
public class Solution43 {
    public static String multiply(String num1, String num2) {
        int len1 = num1.length();
        int len2 = num2.length();
        int []pos = new int [len1+len2];
        for(int i = len1-1; i>=0; i--){
            for(int j = len2-1; j>=0; j--){
                int mul = (num1.charAt(i)-'0')*(num2.charAt(j)-'0');
                int p1 = i + j;
                int p2 = i + j + 1;
                int sum = mul + pos[p2];

                pos[p1]+=sum/10;
                pos[p2]=sum%10;
            }
        }
        StringBuilder str = new StringBuilder();
        for(int p: pos) if(!(str.length() == 0 && p == 0)) str.append(p);
        return str.length() == 0 ? "0" : str.toString();
    }
    public static void main(String[]args){
        System.out.println(multiply("987654321","123456789"));
        // ans = 121932631112635269
    }
}