Restore IP Addresses

class Solution {
public:
    bool IsValid(string &s,int start,int end)
    {
        int sum=0;
        int num=1;
        if(s[start]==‘0‘&&end-start>1)
            return false;
        for(int i=end-1;i>=start;i--)
        {
            sum+=(s[i]-‘0‘)*num;
            num*=10;
        }
        if(sum>=0 && sum<=255)
            return true;
        return false;
    }
    void dfs(vector<string> &result,vector<string> &ip,string &s,int dep)
    {
        if(ip.size()==4)
        {
            if(dep==s.size())
            {
                string addr=ip[0];
                for(int i=1;i<ip.size();i++)
                {
                    addr+="."+ip[i];
                }
                result.push_back(addr);
                return;
            }
        }
        for(int i=1;i<4;i++)
        {
            if(dep+i<=s.size()&&IsValid(s,dep,dep+i))
            {
                int left=s.size()-dep-i;
                if(ip.size()+left/3.0>4)
                    continue;
                ip.push_back(s.substr(dep,i));
                dfs(result,ip,s,dep+i);
                ip.pop_back();
            }
        }
    }
    vector<string> restoreIpAddresses(string s) {
        vector<string> result,ip;
        result.clear();
        ip.clear();
        dfs(result,ip,s,0);
        return result;
    }
};