[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();    }