问这个回文分割算法的时间复杂度是多少？

EN

#include <vector>
using namespace std;

class Solution {
public:
vector<vector<string>> partition(string s) {
    vector<vector<string>> list;
    vector<string> subList;

    // Input validation.
    if (s.length() <= 1) {
        subList.push_back(s);
        list.push_back(subList);
        return list;
    }

    int len = s.length();
    vector<vector<bool>> memo(len, vector<bool>(len));
    for (int i = 0; i < len; i ++) {
        for (int j = 0; j < len; j ++) {
            if (i >= j) memo[i][j] = true;
            else memo[i][j] = false;
        }
    }

    int start = 0;
    helper(s, start, list, subList, memo);

    return list;
}

void helper(string s, int start, 
            vector<vector<string>> &list, vector<string> &subList,
            vector<vector<bool>> &memo) {

    // Base case.
    if (start > s.length() - 1) {
        vector<string> one_rest(subList);
        list.push_back(one_rest);
        return;
    }

    for (int len = 1; start + len <= s.length(); len ++) {
        int end = start + len - 1;

        memo[start][end] = (len == 1) ||
                           (memo[start + 1][end - 1] && s[start] == s[end]);

        if (memo[start][end] == true) {
            // Have a try.
            subList.push_back(s.substr(start, len));

            // Do recursion.
            helper(s, end + 1, list, subList, memo);

            // Roll back.
            subList.pop_back();
        }
    }
}
};