415. Add Strings

题目

Given two non-negative integers num1 andnum2 represented as string, return the sum ofnum1 and num2.

Note:

1. The length of both num1 andnum2 is < 5100.
2. Both num1 andnum2 contains only digits 0-9.
3. Both num1 andnum2 does not contain any leading zero.
4. You must not use any built-in BigInteger library orconvert the inputs to integer directly.

我的解法（有参考答案的优化）

public class Solution {    public String addStrings(String num1, String num2) {        StringBuilder sb = new StringBuilder();        int len1 = num1.length();        int len2 = num2.length();        int mark = 0;        int i = 0;        while(i < len1 || i < len2){            int n1 = (i < len1) ? num1.charAt(len1 - i - 1) - '0' : 0;            int n2 = (i < len2) ? num2.charAt(len2 - i - 1) - '0' : 0;            int num = (n1 + n2 + mark)%10;            sb.append(num);            mark = (n1 + n2 + mark)/10;            i++;        }        return (mark == 0) ? sb.reverse().toString() : sb.append(mark).reverse().toString();    }}

解法分析：使用stringbulider代替string，提高字符串操作速度。使用append()最后再reverse()，而不是Insert(0, num)，因为插入的时间复杂度为o(n^2)，而追加再反转的复杂度为o(n)。