43. Multiply Strings

https://leetcode.com/problems/multiply-strings/discuss/17605/Easiest-JAVA-Solution-with-Graph-Explanation

 

 

class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0") || num2.equals("0")) return "0"; //用equals, 因为可能两个都是"0"
        int[] res = new int[num1.length() + num2.length()];
        int sum = 0, mul = 0;
        for(int i = num1.length() - 1; i >= 0; i--) {
            for(int j = num2.length() - 1; j >= 0; j--) {
                int pos1 = i+j, pos2 = i+j+1;
                mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                sum = mul + res[pos2];
                res[pos1] += sum / 10;   // += 因为之后还要加
                res[pos2] = sum % 10;
            }
        }
        StringBuilder s = new StringBuilder();
        for(int pos : res) {
            if(!(s.length() == 0 && pos == 0)) {
                s.append(pos);
            }
        }
        return s.toString();
        
    }
}