1 解題思路

就是用兩個String表示的數字，不用庫的情況下實現加法。

其實說白了就是高精度加法。。注意進位，注意處理長短不一樣的數字，都從末尾開始相加就好，不多解釋，看程式碼

2 原題

Given two non-negative numbers num1 and num2 represented as string, return the sum of num1 and num2.
Note: 
The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and
 
 num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

3 AC解

public class Solution {
    public String addStrings(String num1, String num2) {
        //交換最大最小的，保證後續
        String longer = num1;
        String shorter = num2;
        if
 
(longer.length() < shorter.length()){
            longer = num2;
            shorter = num1;
        }
        int n=longer.length();
        int m=shorter.length();
        char l[] = longer.toCharArray();
        char s[] = shorter.toCharArray();
        //餘數
        int remainder = 0;
        char
 
 base = '0';
        //從末尾開始加
        int tmp;
        //從末尾開始加
        StringBuilder res = new StringBuilder();
        while(m --> 0){
            n--;
            tmp = l[n] + s[m] - 2*base + remainder;
            remainder = tmp / 10;
            res.append(tmp % 10);
        }
        //處理長的一邊
        while(n--> 0){
            tmp = l[n] - base + remainder;
            remainder = tmp / 10;
            res.append(tmp % 10);
        }
        //處理最後的 進位
        if (remainder !=0 ) res.append(remainder);
        return res.reverse().toString();



    }
}