Solution.java

package _043;

/**
 * <pre>
 *     author: Blankj
 *     blog  : http://blankj.com
 *     time  : 2017/10/17
 *     desc  :
 * </pre>
 */
public class Solution {
    public String multiply(String num1, String num2) {
        if (num1.equals("0") || num2.equals("0")) return "0";
        int l1 = num1.length(), l2 = num2.length(), l = l1 + l2;
        char[] ans = new char[l];
        char[] c1 = num1.toCharArray();
        char[] c2 = num2.toCharArray();
        for (int i = l1 - 1; i >= 0; --i) {
            int c = c1[i] - '0';
            for (int j = l2 - 1; j >= 0; --j) {
                ans[i + j + 1] +=  c * (c2[j] - '0');
            }
        }
        for (int i = l - 1; i > 0; --i) {
            if (ans[i] > 9) {
                ans[i - 1] += ans[i] / 10;
                ans[i] %= 10;
            }
        }
        StringBuilder sb = new StringBuilder();
        int i = 0;
        for (; ; ++i) if (ans[i] != 0) break;
        for (; i < ans.length; ++i) sb.append((char) (ans[i] + '0'));
        return sb.toString();
    }

    public static void main(String[] args) {
        Solution solution = new Solution();
        System.out.println(solution.multiply("132", "19"));
    }
}