Java

public class Solution {
    public String addStrings(String num1, String num2) {
        int tmp=0;
        int k=0;
        int length1=num1.length();
        int length2=num2.length();
        int min=length1>length2? length2:length1;
        int max=length1>length2? length1:length2;
        String use;
        if(length1>length2) 
        use=num1;
        else 
        use=num2;
        String string="";
        int i;
        for(i=0;i<max;)
        {
            int bitAdd;
            if(i<min)
            bitAdd=((int)num1.charAt(length1-1-i)-(int)('0')+(int)num2.charAt(length2-1-i)-(int)('0')+tmp);
            else
            bitAdd=((int)use.charAt(max-1-i)-(int)('0')+tmp);
            
            k=bitAdd%10;
            tmp=bitAdd/10;
            System.out.println(bitAdd);
            string=k+""+string;
            i++;
        }
        if(tmp!=0) string=tmp+""+string;
        return string;
    }
}

Javascript

/**
 * @param {string} num1
 * @param {string} num2
 * @return {string}
 */
var addStrings = function(num1, num2) {
        var tmp=0;
        var k=0;
        var length1=num1.length;
        var length2=num2.length;
        var min=length1>length2? length2:length1;
        var max=length1>length2? length1:length2;
        var use;
        if(length1>length2) 
        use=num1;
        else 
        use=num2;
        var string="";
        var i;
        for(i=0;i<max;)
        {
            var bitAdd;
            if(i<min)
            bitAdd=(parseInt(num1[length1-1-i])+parseInt(num2[length2-1-i])+tmp);
            else
            bitAdd=(parseInt(use[max-1-i])+tmp);
            
            k=bitAdd%10;
            tmp=(bitAdd-k)/10;
            string=k+string;
            i++;
        }
        if(tmp!==0) string=tmp+string;
        return string;
};

注意Javascript做除法不会取整

优解，思路差不多，但是他这个写法很简练

public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        for(int i = num1.length() - 1, j = num2.length() - 1; i >= 0 || j >= 0 || carry == 1; i--, j--){
            int x = i < 0 ? 0 : num1.charAt(i) - '0';
            int y = j < 0 ? 0 : num2.charAt(j) - '0';
            sb.append((x + y + carry) % 10);
            carry = (x + y + carry) / 10;
        }
        return sb.reverse().toString();
    }
}