Today while solving a question on leetcode, I applied dfs on a directed graph which runs on O(N) time, but my code is giving TLE, so after trying too many time I checked on comments and there was a accepted code which also runs on O(N). So now I am confused as why my code is not getting accepted and giving time limit exceeded. My code:-

class Solution {
public:
    string multiply(string num1, string num2) {
        int i=0,j=0;
        int n=0 , m=0;
        while(i<num1.size()){
            if((n>INT_MAX/10) ||(n==INT_MAX/10 && num1[i]>'7') ){
                n=INT_MAX;
            }
            n = n * 10 + (num1[i]-'0');
            ++i;
        }
        while(j<num2.size()){
            if((m>INT_MAX/10) ||(m==INT_MAX/10 && num2[i]>'7') ){
                m=INT_MAX;
            }
            m = m * 10 + (num2[j]-'0');
            ++j;

        }

        long long int result;
        result = m*n;
        string ans;
        ans = to_string(result);
        return ans;
        

        
    }
};

correct my code and give me the correct code