415. Add Strings

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problem

solution

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class Solution {
public:
    string addStrings(string num1, string num2) {
        int n= num1.size(), m = num2.size();
        int len = max(n,m);
        string zero ;
        zero.append(len-min(n,m),'0');
        if(len!=n) num1 = zero + num1;
        if(len!=m) num2 = zero + num2;
        string ret;
        int carry = 0 ;
        for(int i=len-1;i>-1;--i){
            int sum = (num1[i]-'0') + (num2[i]-'0') + carry;
            ret += to_string(sum%10);
            carry = sum/10;
        }
        if(carry) ret +=to_string(carry);
        reverse(ret.begin(), ret.end());
        return ret;
    }
};

analysis

  • time complexity O(n)
  • space complexity O(1)