415. Add Strings

stringbuilder加入到首位的话用insert

 

class Solution {
    public String addStrings(String num1, String num2) {
        if(num1.length() == 0) return num2;
        if(num2.length() == 0) return num1;
        int i = num1.length()-1;
        int j = num2.length()-1;
        int carry = 0;
        StringBuilder sb = new StringBuilder();
        while(i >= 0 || j >= 0){
            if(i < 0){
                int num = (num2.charAt(j) - '0' + carry) % 10;
                carry = (num2.charAt(j) - '0' + carry) / 10;
                sb.insert(0, num);
                j--;
            }else if(j < 0){
                int num = (num1.charAt(i) - '0' + carry) % 10;
                carry = (num1.charAt(i) - '0' + carry) / 10;
                sb.insert(0, num);
                i--;
            }else{
                int num = (num1.charAt(i)- '0' + num2.charAt(j) - '0' + carry) % 10;
                carry = (num1.charAt(i)- '0' + num2.charAt(j) - '0' + carry) / 10;
                sb.insert(0, num);
                i--;
                j--;
            }
        }
        if(carry == 1){
            sb.insert(0, 1);
        }
        return sb.toString();   
        
    }
}