<p>A <strong>valid IP address</strong> consists of exactly four integers separated by single dots. Each integer is between <code>0</code> and <code>255</code> (<strong>inclusive</strong>) and cannot have leading zeros.</p>

<ul>
	<li>For example, <code>&quot;0.1.2.201&quot;</code> and <code>&quot;192.168.1.1&quot;</code> are <strong>valid</strong> IP addresses, but <code>&quot;0.011.255.245&quot;</code>, <code>&quot;192.168.1.312&quot;</code> and <code>&quot;192.168@1.1&quot;</code> are <strong>invalid</strong> IP addresses.</li>
</ul>

<p>Given a string <code>s</code> containing only digits, return <em>all possible valid IP addresses that can be formed by inserting dots into </em><code>s</code>. You are <strong>not</strong> allowed to reorder or remove any digits in <code>s</code>. You may return the valid IP addresses in <strong>any</strong> order.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;25525511135&quot;
<strong>Output:</strong> [&quot;255.255.11.135&quot;,&quot;255.255.111.35&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;0000&quot;
<strong>Output:</strong> [&quot;0.0.0.0&quot;]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;101023&quot;
<strong>Output:</strong> [&quot;1.0.10.23&quot;,&quot;1.0.102.3&quot;,&quot;10.1.0.23&quot;,&quot;10.10.2.3&quot;,&quot;101.0.2.3&quot;]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 20</code></li>
	<li><code>s</code> consists of digits only.</li>
</ul>

class Solution {
private:
    vector<string> result;
    string path = "";
    int iDotCount = 0;
    void backtracking(string s, int startIndex){
        if(iDotCount==4 && startIndex==s.size()){
            path = path.substr(1,path.size()-1);
            result.push_back(path);
            return;
        }
        for(int i=startIndex; i<s.size(); i++){
            string left = s.substr(startIndex, i-startIndex+1);           
            if(isValidIPNum(left)){
                string oldPath = path;
                path = path +"." + left;
                iDotCount++;
                backtracking(s, i+1);
                path = oldPath;
                iDotCount--;
            }
        }
    }
    bool isValidIPNum(string s){
        if(s.size()>3) return false;
        int val = stoi(s);
        if(val>255 || val<0) return false;
        else if (s.size()>1 && s[0]=='0') return false;
        return true;
    }

public:
    vector<string> restoreIpAddresses(string s) {
        backtracking(s,0);
        return result;
    }
};










/*
class Solution {
private:
    vector<string>result;
    string path = "";
    int iDotCount = 0;
    void backtracking(string s,int iStart){
        if(iStart>s.size()) return;
        if(iStart==s.size() && iDotCount==3){
            result.push_back(path);
        }
        for(int i=iStart; i<s.size()&&i<iStart+3; i++){
            string left = s.substr(iStart,i-iStart+1);
            string right = s.substr(i+1,s.size()-1-i);
            if(stoi(left)<256 && (left.size()==1||(left.size()>1 && left[0]!='0'))){
                if(right.size()>(4-iDotCount-1)*3 && right.size()<(4-iDotCount-1)*1) return;
                if(iDotCount>=3) return;
                string oldPath = path;
                int iOldDotCount = iDotCount;
                if(path=="") path = left;
                else {
                    path = path + "." +left;
                    iDotCount++;
                }
                backtracking(s,i+1);
                path = oldPath;
                iDotCount = iOldDotCount;
            }
        }
    }

public:
    vector<string> restoreIpAddresses(string s) {
        result.clear();
        if(s.size()<4 || s.size()>12) return result;
        backtracking(s,0);
        return result;
    }
};
*/
22090