93. Restore IP Addresses

Problem Link

Description

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

• For example, "0.1.2.201" and "192.168.1.1" are valid IP addresses, but "0.011.255.245", "192.168.1.312" and "192.168@1.1" are invalid IP addresses.

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

 

Example 1:

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

Example 2:

Input: s = "0000"
Output: ["0.0.0.0"]

Example 3:

Input: s = "101023"
Output: ["1.0.10.23","1.0.102.3","10.1.0.23","10.10.2.3","101.0.2.3"]

 

Constraints:

• 1 <= s.length <= 20
• s consists of digits only.

Solution

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Python3

class Solution:
    def restoreIpAddresses(self, s: str) -> List[str]:
        N = len(s)
        res = []
 
        def go(index, curr):
            if len(curr) > 4: return
            
            if index == N and len(curr) == 4:
                res.append(".".join(curr))
                return
            
            for k in range(1, 4):
                if index + k > N: break
                word = s[index : index + k]
 
                if (len(word) > 1 and word[0] == "0") or int(word) > 255:
                    continue
                
                go(index + k, curr + [word])
        
        go(0, [])
        return res