415. Add Strings

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

1. The length of both num1 and num2 is < 5100.
2. Both num1 and num2 contains only digits 0-9.
3. Both num1 and num2 does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

字符串表示十进制数，求相加后的结果

C++（19ms）：

class Solution {
public:
    string addStrings(string num1, string num2) {
        int i = num1.length() - 1 ;
        int j = num2.length() - 1 ;
        int c = 0 ;
        string res = "" ;
        while(i >= 0 || j >= 0 || c == 1){
            if (i >= 0)
                c += num1[i--] - '0' ;
            if (j >= 0)
                c += num2[j--] - '0' ;
            res = to_string(c%10) + res ;
            c/=10 ;
        }
        return res ;
    }
};

java(25ms):

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