[LeetCode] Restore IP Addresses

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)

#define pb push_back
#define pp pop_back
class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
    	vector<string> ret;
		string cur;
		solve(s,0,1,cur,ret);
		return ret;
    }
	void solve(string& s,int i,int kth,string& cur,vector<string>& ret)
	{
		int len=s.length();
		if ( i>=len || kth>4 )
		{
			if ( i==len && kth==5)
				ret.pb(cur);
			return;
		}
		cur.pb(s[i]);
		if (kth <4 )
			cur.pb('.');
		solve(s,i+1,kth+1,cur,ret);
		if (kth<4 )
			cur.pp();
		cur.pp();

		if ( s[i]=='0' )
			return;
		//two digits
		if ( i<len-1 )
		{
			cur.pb(s[i]);
			cur.pb(s[i+1]);
			if (kth<4)
				cur.pb('.');
			solve(s,i+2,kth+1,cur,ret);
			if ( kth<4)
				cur.pp();
			cur.pp();
			cur.pp();
		}

		if (i <len-2)
		{
			int num= (s[i]-'0')*100+(s[i+1]-'0')*10+(s[i+2]-'0');
			if ( num>255 )
				return;
			cur.pb(s[i]);
			cur.pb(s[i+1]);
			cur.pb(s[i+2]);
			if ( kth<4 )
				cur.pb('.');
			solve(s,i+3,kth+1,cur,ret);
			if ( kth<4 )
				cur.pp();
			cur.pp();
			cur.pp();
			cur.pp();
		}
	}
};