[开发工具][LeetCode] Add Strings

Description

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

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

Solution

The basic idea is to add the numbers one by one from tail to head.

Code

class Solution {
public:
    string addStrings(string num1, string num2) {
        int l1 = num1.length(), l2 = num2.length(), flag = 0;
        string res = "";
        while(l1 + l2 + flag) {
            int sum = 0;
            if(l1 == 0 && l2 == 0)
                sum = sum + flag;
            else if(l1 == 0) 
                sum = sum + int(num2[--l2] - '0') + flag;
            else if(l2 == 0) 
                sum = sum + int(num1[--l1] - '0') + flag;
            else 
                sum = sum + int(num1[--l1] - '0') + int(num2[--l2] - '0') + flag;
            
            flag = sum/10;
            res = char(sum%10 + '0') + res;
        }
        return res;
    }
};

Complexity

Time complexity: O(max(m,n))
Space complexity: O(max(m,n))