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# 415.Add-Strings

## 415. Add Strings

## 题目地址

<https://leetcode.com/problems/add-strings/>

## 题目描述

```
Given two non-negative integers 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.
```

## 代码

### Approach #1

```java
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((s + y + carry) % 10);
      carry = (x + y + carry) / 10;
    }

    return sb.reverse().toString();
  }
}
```