Given two non-negative numbers 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.

很简单的一个题目

这是我自己写的一个做法，有点冗余

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public String addStrings(String num1, String num2) {
    int m = num1.length();
    int n = num2.length();
    StringBuilder sb = new StringBuilder();
    int carry = 0;
    int i = 0;
    for (; i < m && i < n; i++) {
        int b = num1.charAt(m - i - 1) - '0' + num2.charAt(n - i - 1) - '0' + carry;
        carry = b / 10;
        sb.insert(0, b % 10);
    }
    for (; i < m; i++) {
        int b = num1.charAt(m - i - 1) - '0' + carry;
        carry = b / 10;
        sb.insert(0, b % 10);
    }
    for (; i < n; i++) {
        int b = num2.charAt(n - i - 1) - '0' + carry;
        carry = b / 10;
        sb.insert(0, b % 10);
    }
    if (carry == 1)
        sb.insert(0, carry);
    return sb.toString();
}

Discuss里面比较简短的一个

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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; 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;
    }
    if (carry != 0)
        sb.append(carry);
    return sb.reverse().toString();
}