Restore IP Addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

思路：這是一道DFS題，在給字符串截取分配時，每次可以截一個、2個、三個，然後判斷截取這三個是否滿足ip的一個段，然後繼續下一層。到達葉子節點時，需要判斷ip段是否到達4並且字符串到達尾部，則從根節點到葉子節點是ip分段的一種。

class Solution {
private:
    vector<string> res;
    vector<string> perRes;
public:
    int stringToInt(string s) {
        int len = s.length();
        int i,j=1,res=0;
        for(i=len-1; i>=0; i--) {
            res += (s[i]-'0')*j;
            j *= 10;
        }
        return res;
    }
    string stringToIp(vector<string> aa)
    {
        int size = aa.size();
        string str = "";
        int i;
        for(i=0; i<size; ++i)
        {
            str += aa[i];
            if (i != size-1)
            {
                str += '.';
            }    
        }    
        return str;
    }    
    void restoreIpAddressesHelper(string s,int start, int end, int ipSeg) {
        if (start >= end-1 && ipSeg == 4) {
            res.push_back(stringToIp(perRes));
            return;
        }
        if (start >= end-1 && ipSeg != 4) {
            return;
        }
        if (start < end && ipSeg == 4) {
            return;
        }
        int i, strInt,j;
        for(j=1; j<=3; ++j)
        {
            if (start + j < end)
            {
                string str = s.substr(start+1,j);
                strInt = stringToInt(str);
                if ((j==1&&strInt>=0&&strInt<=9)||(j==2&&strInt>=10&&strInt<=99)||(j==3&&strInt>=100&&strInt<=255)) {
                    perRes[ipSeg] = str;
                    restoreIpAddressesHelper(s,start+j,end,ipSeg+1);
                }
            }    
        }    
    }
    vector<string> restoreIpAddresses(string s) {
        int len = s.length();
        res.clear();
        perRes.clear();
        perRes.resize(4);
        if (len < 4) {
            return res;
        }
        restoreIpAddressesHelper(s,-1,len,0);
        return res;
    }
};