Leetcode 415. Add Strings

Leetcode 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.

思路： 字符串模拟加法

class Solution {

    public String addStrings(String num1, String num2) {
        if(num1.length() < num2.length()) { // 让num1始终为较长的那个字符串
            String temp = num1;
            num1 = num2;
            num2 = temp;
        }
        int len1 = num1.length();
        int len2 = num2.length();
        num1 = new StringBuilder(num1).reverse().toString(); //逆序
        num2 = new StringBuilder(num2).reverse().toString();
        StringBuilder sb = new StringBuilder();
        int carry = 0; // 进位
        for(int i = 0 ; i < len1; i++) {
            int a = num1.charAt(i) - '0';
            int b = (i < len2) ? num2.charAt(i) - '0' : 0;
            int r = a + b + carry; // 别忘了加上进位
            if(r >= 10) {
                r %= 10; 
                carry = 1;
            }else {
                carry = 0;
            }
            sb.append(r);
        } 
        if(carry == 1) sb.append(carry); // 如果最后还有进位，别忘了加上
        return sb.reverse().toString();
    }
}