Question

link

Stats

Frequency	3
Difficulty	3
Adjusted Difficulty	3
Time to use	----------

Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)

Analysis

This is question can be solved by either DFS or Brute Force, both are fine.

Solution

The DFS solution is obvious, I have the code for it.

However, there’s another person who wrote much less code while implementing the same solution as mine. I will post his code as a good example to learn.

The brute force solution in this case does not sound like a bad idea. This is the most top-rated idea on official forum as well.

You can get points' positions by i, j, k. Using these positions, you can divide s into candidate ip-form. Then, you can judge whether the candidate fits ip. To improve the efficiency, you can narrow the scope of i, j, k.

So, this BF code is also posted below.

Just one more thing. I tested the exact same BF code written in C++. Compared to the 420ms it took Java to pass OJ test, C++ takes 8ms only! I was a bit shocked.

Code

First, DFS, my code

Note: I could have just insert as string, so that convert() method would not be needed.

public ArrayList<String> restoreIpAddresses(String s) {
    ArrayList<String> ans = new ArrayList<String>();
    helper(ans, new ArrayList<String>(), s, 0);
    return ans;
}

private void helper(ArrayList<String> ans, ArrayList<String> cur, String s, int from) {
    int len = s.length();
    if (from >= len) return;
    if (cur.size() == 3) {
        String lastStr = s.substring(from);
        if (isValidIpNumber(lastStr)) {
            ArrayList<String> oneAns = new ArrayList<String>(cur);
            oneAns.add(lastStr);
            ans.add(convert(oneAns));
        }
    }
    else {
        // cur.size less than 3, so get next num (length = 1, 2 or 3)
        for (int i = 1; i <= 3 && from + i <= len; i ++) {
            String nextStr = s.substring(from, from + i);
            if (isValidIpNumber(nextStr)) {
                cur.add(nextStr);
                helper(ans, cur, s, from + i);
                cur.remove(cur.size() - 1);
            }
        }
    }
}

private boolean isValidIpNumber(String str) {
    if (str.length() == 0 || str.length() > 3) return false;
    if (str.charAt(0) == '0' && str.length() != 1) return false;
    int num = Integer.parseInt(str);
    return (0 <= num && num <= 255);
}

private String convert(ArrayList<String> l) {
    String ans = "";
    for (String a: l)
        ans += "." + a;
    return ans.substring(1);
}

Second, DFS, shorter version code

public ArrayList<String> restoreIpAddresses(String s) {
    ArrayList<String> res = new ArrayList<String>();  
    if (s.length()<4||s.length()>12) return res;  
    dfs(s,"",res,0);  
    return res;  
}  

public void dfs(String s, String tmp, ArrayList<String> res, int count){  
    if (count == 3 && isValid(s)) {  
        res.add(tmp + s);  
        return;  
    }  
    for(int i=1; i<4 && i<s.length(); i++){  
        String substr = s.substring(0,i);  
        if (isValid(substr)){  
            dfs(s.substring(i), tmp + substr + '.', res, count+1);  
        }  
    }  
}  

public boolean isValid(String s){  
    if (s.charAt(0)=='0') return s.equals("0");  
    int num = Integer.parseInt(s);  
    return num<=255 && num>0;  
}  

Third, BF code using triple nested loop (Java)

public ArrayList<String> restoreIpAddresses(String s) {
    ArrayList<String> res = new ArrayList<String>();  
    if (s.length() > 12 || s.length() < 4) return res;
    for (int i = 1; i < 4; i ++) {
        String first = s.substring(0, i);
        if (! isValid(first)) continue;
        for (int j = 1; i + j < s.length() && j < 4; j ++) {
            String second = s.substring(i, i + j);
            if (! isValid(second)) continue;
            for (int k = 1; i + j + k < s.length() && k < 4; k ++) {
                String third = s.substring(i + j, i + j + k);
                String fourth = s.substring(i + j + k);
                if (isValid(third) && isValid(fourth)) 
                    res.add(first + "." + second + "." + third + "." + fourth);
            }
        }
    }
    return res;
}  

public boolean isValid(String s) {
    if (s.length() > 1 && s.charAt(0) == '0') return false;
    return 0 <= Integer.parseInt(s) && Integer.parseInt(s) <= 255;  
}

Fourth, same BF code (C++)

vector<string> restoreIpAddresses(string s) {
    vector<string> res;
    if (s.size() > 12 || s.size() < 4) return res;
    for (int i=1; i<4; i++) {
        string first = s.substr(0, i);
        if (!isValid(first)) continue;
        for (int j=1; i+j < s.size() && j<4; j++) {
            string second = s.substr(i, j);
            if (!isValid(second)) continue;
            for (int k=1; i+j+k < s.size() && k<4; k++) {
                string third = s.substr(i+j, k);
                string fourth = s.substr(i+j+k);
                if (isValid(third) && isValid(fourth)) {
                    string temp = first+"."+second+"."+third+"."+fourth;
                    res.push_back(temp);
                }
            }
        }
    }
    return res;
}

bool isValid(string s) {
    if (s.size() > 1 && s[0] == '0') return false;
    if (stoi(s) <= 255 && stoi(s) >= 0) return true;
    else return false;
}