LeetCode 415. Add Strings

题目

Given two non-negative integers 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.

思考

既然题目不能用将string换成int的函数，而且返回的和也为string，就直接用string的字符来进行加法操作。
分4种情况：
1. 其中一个为空（或为0），答案及为另一个数；
2. 二者长度一样；
3. sum1比sum2长；
4. sum1比sum2短。

答案

c++

class Solution {public:    string addStrings(string num1, string num2) {        if (num1.length() == 0) return num2;        if (num2.length() == 0) return num1;        string a(num1), b(num2);        reverse(a.begin(), a.end());        reverse(b.begin(), b.end());        string result = "";        int len1 = num1.length(), len2 = num2.length();        int temp = len1 < len2 ? len1 : len2;        int i;        result = a[0] + b[0] - '0';        for (i = 1; i < temp; i++) {            char add = a[i] + b[i] - '0';            if (result[i-1] > '9') {                add++;                result[i-1] -= 10;            }            result += add;        }        if (len1 < len2) {            for (; i < len2; i++) {                char add = b[i];                if (result[i-1] > '9') {                    add++;                    result[i-1] -= 10;                }                result += add;            }        } else if (len1 > len2) {            for (; i < len1; i++) {                char add = a[i];                if (result[i-1] > '9') {                    add++;                    result[i-1] -= 10;                }                result += add;            }        }        if (result[i-1] > '9') {            result += '1';            result[i-1] -= 10;        }        reverse(result.begin(), result.end());        return result;    }};