自己构建cmp算子

179. Largest Number

class Solution {
public:
    string largestNumber(vector& nums) {
        vector numstr;
        for (int n : nums) {
            numstr.push_back(to_string(n));
        }
        sort(numstr.begin(), numstr.end(), [this](string lhs, string rhs) { return cmpf(lhs, rhs); });
        string res = accumulate(numstr.begin(), numstr.end(), string(""));
        return res.find_first_not_of('0') == string::npos ? "0" : res;
    }
private:
    bool cmpf(string lhs, string rhs) {
        if (lhs.length() == rhs.length()) {
             return lhs > rhs;
         }
         int m = lhs.length(), n = rhs.length();
         int cmp = memcmp(lhs.data(), rhs.data(), min(m, n));
         if (cmp != 0) {
             return cmp > 0;
         }
         return m < n ? cmpf(lhs, rhs.substr(m)) : cmpf(lhs.substr(n), rhs);
    }
};