HackMD
93.Restore IP Addresses
tags: Medium,String,Backtracking
題目描述
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.
範例
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 sconsists of digits only.
解答
C++
作法
- backtracking走一遍
class Solution {
public:
vector<string> ans;
void BT(const string &s, string address = "", string cur = "", int i = 0) {
if (i == s.size()) {
if (cur.empty() && count(address.begin(), address.end(), '.') == 3) {
ans.push_back(address);
}
return;
}
if (!cur.empty() && stoi(cur) == 0) return;
cur.push_back(s[i]);
if (stoi(cur) > 255) return;
BT(s, address, cur, i + 1);
if (address.empty()) {
BT(s, cur, "", i + 1);
} else {
BT(s, address + "." + cur, "", i + 1);
}
}
vector<string> restoreIpAddresses(string s) {
BT(s);
return ans;
}
};
Yen-Chi ChenSat, Jan 21, 2023
Python
class Solution:
def restoreIpAddresses(self, s: str) -> List[str]:
ans = []
def BT(address = [int(s[0])], i = 1):
if i == len(s):
if len(address) == 4:
ans.append('.'.join(map(str, address)))
return
old = address[-1]
new = old * 10 + int(s[i])
if old != 0 and new <= 255:
address[-1] = new
BT(address, i + 1)
address[-1] = old
if len(address) < 4:
BT(address + [int(s[i])], i + 1)
BT()
return ans
Yen-Chi ChenSat, Jan 21, 2023
class Solution:
def restoreIpAddresses(self, s: str) -> List[str]:
def check0_255(s):
x = int(s)
return x>0 and x<=255
def dfs(_s, pnts):
#print('##', _s, pnts)
if len(_s) > 3*(pnts+1) or len(_s) == 0:
return []
if pnts == 0:
#print('##', pnts, _s, len(_s))
if len(_s) > 4 or len(_s) == 0:
return []
if _s == '0':
return [['0']]
if _s[0] == '0':
return []
if check0_255(_s):
return [ [_s] ]
return []
if _s[0] == '0':
result = []
for item in dfs(_s[1:], pnts-1):
#print(item)
result.append( ['0'] + item)
return result
result = []
for digit in [1,2,3]:
x = int( _s[0:digit] )
#print(pnts, x)
if check0_255(x):
for item in dfs(_s[digit:], pnts-1):
#print(pnts, _s, item)
result.append( [str(x)] + item)
#print(pnts, _s, result)
return result
Q = dfs(s, 3)
return [ ".".join(x) for x in Q]
