LeetCode Largest Number

class Cmp {
public:
bool operator() (const string& a, const string& b) {
return cmp(a + b, b + a, a.size() + b.size());
}

bool cmp(const string a, const string b, int len) {
int pos = 0;
while (pos < len) {
if (a[pos] != b[pos]) {
return a[pos] < b[pos];
}
pos++;
}
return false;
}
};
class Solution {
public:
string largestNumber(vector<int>& nums) {
vector<string> strnums;
for (int num : nums) {
strnums.push_back(num2str(num));
}

sort(strnums.begin(), strnums.end(), Cmp());

string res;
int len = strnums.size();
for (int i=len - 1; i>=0; i--) {
if (res.size() == 0 && strnums[i][0] == '0') {
continue;
}
res = res + strnums[i];
}
if (res.size() == 0) {
res = "0";
}
return res;
}

string num2str(int num) {
char buf[20] = {0};
sprintf(buf, "%d", num);
return string(buf);
}
};