[LeetCode] 93. Restore IP Addresses 复原IP地址

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

Example:

Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]

解法：Backtracking

Java:

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<String>();
        int len = s.length();
        for(int i = 1; i<4 && i<len-2; i++){
            for(int j = i+1; j<i+4 && j<len-1; j++){
                for(int k = j+1; k<j+4 && k<len; k++){
                    String s1 = s.substring(0,i), s2 = s.substring(i,j), s3 = s.substring(j,k), s4 = s.substring(k,len);
                    if(isValid(s1) && isValid(s2) && isValid(s3) && isValid(s4)){
                        res.add(s1+"."+s2+"."+s3+"."+s4);
                    }
                }
            }
        }
        return res;
    }
    public boolean isValid(String s){
        if(s.length()>3 || s.length()==0 || (s.charAt(0)=='0' && s.length()>1) || Integer.parseInt(s)>255)
            return false;
        return true;
    }
}　　

Java:

public List<String> restoreIpAddresses(String s) {
    List<String> solutions = new ArrayList<String>();
    restoreIp(s, solutions, 0, "", 0);
    return solutions;
}

private void restoreIp(String ip, List<String> solutions, int idx, String restored, int count) {
    if (count > 4) return;
    if (count == 4 && idx == ip.length()) solutions.add(restored);

    for (int i=1; i<4; i++) {
        if (idx+i > ip.length()) break;
        String s = ip.substring(idx,idx+i);
        if ((s.startsWith("0") && s.length()>1) || (i==3 && Integer.parseInt(s) >= 256)) continue;
        restoreIp(ip, solutions, idx+i, restored+s+(count==3?"" : "."), count+1);
    }
}　　

Java:

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<String>();
        for (int a = 1; a < 4; ++a)
        for (int b = 1; b < 4; ++b)
        for (int c = 1; c < 4; ++c)
        for (int d = 1; d < 4; ++d)
            if (a + b + c + d == s.length()) {
                int A = Integer.parseInt(s.substring(0, a));
                int B = Integer.parseInt(s.substring(a, a + b));
                int C = Integer.parseInt(s.substring(a + b, a + b + c));
                int D = Integer.parseInt(s.substring(a + b + c));
                if (A <= 255 && B <= 255 && C <= 255 && D <= 255) {
                    String t = String.valueOf(A) + "." + String.valueOf(B) + "." + String.valueOf(C) + "." + String.valueOf(D);
                    if (t.length() == s.length() + 3) res.add(t);
                }
            }
        return res;
    }
}

Java:

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<String>();
        helper(s, 0, "", res);
        return res;
    }
    public void helper(String s, int n, String out, List<String> res) {
        if (n == 4) {
            if (s.isEmpty()) res.add(out);
            return;
        }
        for (int k = 1; k < 4; ++k) {
            if (s.length() < k) break;
            int val = Integer.parseInt(s.substring(0, k));
            if (val > 255 || k != String.valueOf(val).length()) continue;
            helper(s.substring(k), n + 1, out + s.substring(0, k) + (n == 3 ? "" : "."), res);
        }
    }
}

　　

　　

Python:

class Solution:
    # @param s, a string
    # @return a list of strings
    def restoreIpAddresses(self, s):
        result = []
        self.restoreIpAddressesRecur(result, s, 0, "", 0)
        return result

    def restoreIpAddressesRecur(self, result, s, start, current, dots):
        # pruning to improve performance
        if (4 - dots) * 3 < len(s) - start or (4 - dots) > len(s) - start:
            return

        if start == len(s) and dots == 4:
            result.append(current[:-1])
        else:
            for i in xrange(start, start + 3):
                if len(s) > i and self.isValid(s[start:i + 1]):
                    current += s[start:i + 1] + '.'
                    self.restoreIpAddressesRecur(result, s, i + 1, current, dots + 1)
                    current = current[:-(i - start + 2)]

    def isValid(self, s):
        if len(s) == 0 or (s[0] == '0' and s != "0"):
            return False
        return int(s) < 256

C++:

vector<string> restoreIpAddresses(string s) {
        vector<string> ret;
        string ans;

        for (int a=1; a<=3; a++)
        for (int b=1; b<=3; b++)
        for (int c=1; c<=3; c++)
        for (int d=1; d<=3; d++)
            if (a+b+c+d == s.length()) {
                int A = stoi(s.substr(0, a));
                int B = stoi(s.substr(a, b));
                int C = stoi(s.substr(a+b, c));
                int D = stoi(s.substr(a+b+c, d));
                if (A<=255 && B<=255 && C<=255 && D<=255)
                    if ( (ans=to_string(A)+"."+to_string(B)+"."+to_string(C)+"."+to_string(D)).length() == s.length()+3)
                        ret.push_back(ans);
            }    

        return ret;
    }

　　

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[LeetCode] 17. Letter Combinations of a Phone Number 电话号码的字母组合

　　

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