Add Strings

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415.Add Strings

Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.

You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.

Example 1:

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Input: num1 = "11", num2 = "123"
Output: "134"

Example 2:

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Input: num1 = "456", num2 = "77"
Output: "533"

Example 3:

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Input: num1 = "0", num2 = "0"
Output: "0"

时间复杂度：O(max(len1,len2)

空间复杂度：O(1)

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class Solution {
public:
    string addStrings(string num1, string num2) {
        int i = num1.length() - 1, j = num2.length() - 1, add = 0;
        string ans = "";
        while (i >= 0 || j >= 0 || add != 0) {
            int x = i >= 0 ? num1[i] - '0' : 0;
            int y = j >= 0 ? num2[j] - '0' : 0;
            int result = x + y + add;
            ans.push_back('0' + result % 10);
            add = result / 10;
            i -= 1;
            j -= 1;
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};