Leetcode Multiply Strings

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.

---

解题思路：

没想出来，看的答案。

The key to solve this problem is multiplying each digit of the numbers at the corresponding positions and get the sum values at each position. That is how we do multiplication manually.

用StringBuilder 也有不同的用法，可以用insert , 也可以用append. 不过用append后面要reverse(), 因此run time 增加了。

---

Java code:

public String multiply(String num1, String num2) {
        String n1 = new StringBuilder(num1).reverse().toString();
        String n2 = new StringBuilder(num2).reverse().toString();
        int[] d = new int[num1.length() + num2.length()];    
        
        //multiply each digit and sum at the corresponding positions
        for(int i=0; i < n1.length(); i++){
            for(int j=0; j < n2.length(); j++){
                d[i+j] += (n1.charAt(i) - '0') *(n2.charAt(j) - '0');
            }
        }
        
        StringBuilder sb = new StringBuilder();
        
        //calculate each digit
        for(int i=0; i< d.length; i++){
            int mod = d[i]%10;
            int carry = d[i]/10;
            if(i+1 < d.length){
                d[i+1] += carry;
            }
            sb.insert(0, mod);
        }
        
        //remove front 0's
        while(sb.charAt(0) == '0' && sb.length() >1) {
            sb.deleteCharAt(0);
        }
        return sb.toString();
    }

或者

public String multiply(String num1, String num2) {
        String n1 = new StringBuilder(num1).reverse().toString();
        String n2 = new StringBuilder(num2).reverse().toString();
        int[] d = new int[num1.length() + num2.length()];    
        
        //multiply each digit and sum at the corresponding positions
        for(int i=0; i < n1.length(); i++){
            for(int j=0; j < n2.length(); j++){
                d[i+j] += (n1.charAt(i) - '0') *(n2.charAt(j) - '0');
            }
        }
        
        StringBuilder sb = new StringBuilder();
        
        //calculate each digit
        for(int i=0; i< d.length; i++){
            int mod = d[i]%10;
            int carry = d[i]/10;
            if(i+1 < d.length){
                d[i+1] += carry;
            }
            sb.append(mod);
        }
        
        //remove front 0's
        while(sb.length() >1 && sb.charAt(sb.length()-1) == '0') {
            sb.deleteCharAt(sb.length()-1);
        }
        return sb.reverse().toString();
    }

---

Reference:

1. http://www.programcreek.com/2014/05/leetcode-multiply-strings-java/