MultiplyStrings.java

/*
 Author:     King, wangjingui@outlook.com
 Date:       Dec 20, 2014
 Problem:    Multiply Strings
 Difficulty: Easy
 Source:     https://oj.leetcode.com/problems/multiply-strings/
 Notes:
 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: Just like what we do when multiplying integers.
 */
public class Solution {
    public String multiply(String num1, String num2) {
        int l1 = num1.length(), l2 = num2.length();
        if (l1 == 0 || l2 == 0) return new String("");
        if (num1.charAt(0) == '0' || num2.charAt(0) == '0') return new String("0");
        StringBuffer sb = new StringBuffer();
        int[] res = new int[l1+l2];
        for (int i = 0; i < l1; ++i) {
            for (int j = 0; j < l2; ++j) {
                res[i+j+1] += (num1.charAt(i)-'0') *(num2.charAt(j)-'0');
            }
        }
        int c = 0;
        for (int i = res.length - 1; i>=1; --i) {
            res[i] += c;
            c = res[i] / 10;
            res[i] = res[i] % 10;
            sb.insert(0,res[i]);
        }
        if (c !=0 || res[0] != 0) {
            sb.insert(0,c+res[0]);
        }
        return sb.toString();
    }
}