MultiplyStrings.java

package multiplyStrings;


public class MultiplyStrings {
	public String multiply(String num1, String num2) {
		if(num1.equals("0") || num2.equals("0")){
			return "0";
		}
		String result = "0";
        if(num1.length() < num2.length()){
        	String tmp = null;
        	tmp = num1;
        	num1 = num2;
        	num2 = tmp;
        }
        for(int i = num2.length()-1; i >= 0; i--){
        	char rate = num2.charAt(i);
        	String tmp = multiply(num1, rate, num2.length()-1-i);
        	result = add(tmp, result);
        }
		result = rotate(result);
		return result;
    }
	
	public String multiply(String multi, char rate, int round){
		String result = "";
		int a = (int)rate - 48;
		int front = 0;
		for(int i = 0; i < round; i++){
			result += "0";
		}
		for(int i = multi.length()-1; i >= 0; i--){
			char mul = multi.charAt(i);
			int b = (int)mul - 48;
			int middleResult = a*b + front;
			int middleString = middleResult%10;
			front = middleResult/10;
			result += String.valueOf(middleString);
		}
		if(front != 0){
			result += String.valueOf(front);
		}
		return result;
	}
	
	public String add(String add1, String add2){
		//×Ö·ûÊÇ·´¹ýÀ´µÄ
		if(add2.equals("0")){
			return add1;
		}
		if(add1.equals("0")){
			return add2;
		}
		String result = "";
		//the length of add1 >= add2
		int front = 0;
		for(int i = 0; i < add2.length(); i++){
			int a = (int)add2.charAt(i)-48;
			int b = (int)add1.charAt(i)-48;
			int sum = a+b+front;
			if(sum >= 10){
				front = 1;
				sum = sum - 10;
			}
			else{
				front = 0;
			}
			result += String.valueOf(sum);
		}
		if(front == 1){
			if(add1.length() - add2.length() == 0){
				result += "1";
			}
			else {
				String tmp = add1.substring(add2.length(), add1.length());
				String tmpResult = add(tmp, "1");
				result += tmpResult;
			}
		}
		else{
			result += add1.substring(add2.length(), add1.length());
		}
		
		return result;
	}
	
	public String rotate(String cur){
		char[] tmp = cur.toCharArray();
		String result = "";
		for(int i = tmp.length-1; i >= 0; i--){
			result += tmp[i];
		}
		return result;
	}
	
	public static void main(String[] args){
		MultiplyStrings t = new MultiplyStrings();
		String a = "4806";
		String b = "896";
		String result = t.multiply(a, b);
		System.out.println(result);
	}
}