我嘞个TLE

#include <iostream>
#include <algorithm>
#include <string>
#include <cstring>
using namespace std;
string add(string m,string n)
{
    string ans;
    int ml = m.size(),nl = n.size();
    int num1[1000001],numsum[1000001],num2[1000002];
    memset(num1,0,sizeof(num1));
    memset(num2,0,sizeof(num2));
    memset(numsum,0,sizeof(numsum));
    while(m[ml] == '0' && ml > 1) ml--;
    while(n[nl] == '0' && nl > 1) nl--;
    for(int i = 1; i <= ml; i++)num1[i] = m[ml - i] - '0';
    for(int i = 1; i <= nl; i++)num2[i] = n[nl - i] - '0';
    int maxlen = max(nl,ml)+ 1;
    for(int i = 1; i <= maxlen; i++)
    {
        numsum[i] += num2[i] + num1[i];
        numsum[i + 1] += numsum[i] / 10;
        numsum[i] %= 10;
    }
    if(numsum[maxlen] == 0) maxlen--;
    for(int i = maxlen; i >= 1; i--) ans += numsum[i] + '0';
    return ans;
}
int main()
{
    int _;
    cin >> _;
    string s[5001] = {"1","1"};
    for(int i= 2; i <= _; i++)
    {
        s[i] = add(s[i - 1],s[i - 2]);
    }
    cout << s[_];
}