[LeetCode]--415. Add Strings

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.

public String addStrings(String num1, String num2) {
        char[] num1s = num1.toCharArray();
        char[] num2s = num2.toCharArray();
        int flag = 0, temp = 0, len1 = num1s.length - 1, len2 = num2s.length - 1;
        String result = "";
        int i = len1, j = len2;
        for (; i >= 0 && j >= 0; i--, j--) {
            temp = num1s[i] - '0' + num2s[j] - '0' + flag;
            if (temp > 9) {
                flag = 1;
                temp = temp % 10;
            } else
                flag = 0;
            result = temp + result;
        }
        while (i >= 0) {
            temp = num1s[i] - '0' + flag;
            if (temp > 9) {
                flag = 1;
                temp = temp % 10;
            } else
                flag = 0;
            result = temp + result;
            i--;
        }
        while (j >= 0) {
            temp = num2s[j] - '0' + flag;
            if (temp > 9) {
                flag = 1;
                temp = temp % 10;
            } else
                flag = 0;
            result = temp + result;
            j--;
        }
        if (flag == 1)
            result = flag + result;
        return result;
    }

看别人简便的写法，只是语言结构上简便。

public String addStrings(String num1, String num2) {
        StringBuffer sb = new StringBuffer();
        int l1 = num1.length();
        int l2 = num2.length();
        char[] c1 = num1.toCharArray();
        char[] c2 = num2.toCharArray();
        int carry = 0;
        for (int i = 0; i < l1 || i < l2; i++) {
            int a1 = i < l1 ? c1[l1 - 1 - i] - 48 : 0;
            int a2 = i < l2 ? c2[l2 - 1 - i] - 48 : 0;
            int ans = a1 + a2 + carry;
            sb.append(ans % 10);
            carry = ans / 10;
        }
        if (carry == 1) {
            sb.append("1");
        }
        return sb.reverse().toString();
    }