_043.java

public class Solution {
    public String multiply(String num1, String num2) {
        int n1 = num1.length();
        int n2 = num2.length();
        int[] ret = new int[n1+n2];
        //memset(ret,0,sizeof(ret));
        for(int i = n1-1;i >= 0;i--){
            for(int j = n2-1;j >= 0;j--){
                int a1 = num1.charAt(i)-'0';
                int a2 = num2.charAt(j)-'0';
                int tmp = a1*a2;
                int location = n1-i+n2-j-2;
                ret[location] += tmp;
            }
        }
        for(int i = 0;i < n1+n2-1;i++){
            ret[i+1] += ret[i]/10;
            ret[i] = ret[i]%10;
        }
        StringBuilder t = new StringBuilder();
        boolean flag = false;
        for(int i = n1+n2-1;i >=0;i--){
            if(!flag&&ret[i] == 0){
                //skip
            }
            else{
                flag = true;
                //char put = '0'+ret[i];
                t.append(ret[i]);
            }
        }
        String re = t.toString();
        if(re.length() == 0) re = "0";
        //reverse(re.begin(),re.end());
        return re;
    }
}