LeetCode: Restore IP Addresses

class Solution {
public:
void dfs(vector<string> &ret, string tmp, string s, int dep, int maxsize, int beg) {
if (beg > maxsize) return;
if (dep == 4) {
if (beg == maxsize) {
tmp.erase(beg+3, 1);
ret.push_back(tmp);
}
return;
}
dfs(ret, tmp+s[beg]+'.',s, dep+1, maxsize, beg+1);
if (s.substr(beg, 2) >= "10" && s.substr(beg, 2) <= "99")
dfs(ret, tmp+s.substr(beg, 2)+'.', s, dep+1, maxsize, beg+2);
if (s.substr(beg, 3) >= "100" && s.substr(beg, 3) <= "255")
dfs(ret, tmp+s.substr(beg, 3)+'.', s, dep+1, maxsize, beg+3);
}
vector<string> restoreIpAddresses(string s) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<string> ret;
string tmp = "";
dfs(ret, tmp, s, 0, s.size(), 0);
return ret;
}
};

public class Solution {
public List<string> RestoreIpAddresses(string s) {
List<string> ans = new List<string>();
string tmp = "";
dfs(ref ans, tmp, s, 0, s.Length, 0);
return ans;
}
public void dfs(ref List<string> ans, string tmp, string s, int dep, int maxSize, int beg) {
if (beg > maxSize) return;
if (dep == 4) {
if (beg == maxSize) {
ans.Add(tmp.Substring(0, tmp.Length-1));
}
return;
}
if (beg < maxSize) dfs(ref ans, tmp + s[beg] + '.', s, dep + 1, maxSize, beg + 1);
if (beg + 2 <= maxSize && string.Compare(s.Substring(beg, 2), "10") >= 0 && string.Compare(s.Substring(beg, 2), "99") <= 0)
dfs(ref ans, tmp + s.Substring(beg, 2) + '.', s, dep + 1, maxSize, beg + 2);
if (beg + 3 <= maxSize && string.Compare(s.Substring(beg, 3), "100") >= 0 && string.Compare(s.Substring(beg, 3), "255") <= 0)
dfs(ref ans, tmp + s.Substring(beg, 3) + '.', s, dep + 1, maxSize, beg + 3);
}
}