[LeetCode]--415. Add Strings-优快云博客

本文介绍了一种不使用内置大数库或直接转换为整数的方法来实现两个非负数字符串的相加操作。通过两种不同的实现方式，展示了如何逐位计算并处理进位的过程。

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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();
    }