Restore IP Addresses

Description

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.

Code

可以學習的地方在於：num = num * 10 + (s[j] - '0');

不需要用字串去比較，也不需要使用 stoi 之類的。

class Solution {
public:
    vector<string> res;
    vector<string> restoreIpAddresses(string s) {
        dfs(s, 0, "", 0);
        return res;
    }
 
    void dfs(string& s, int i, string cur, int n) {
        if(i == s.length() && n == 4) {
            cur.pop_back(); // pop the last "."
            res.push_back(cur);
            return;
        }  
 
        int num = 0;
        for(int j = i; j < i + 3; j++) {
            if(j >= s.length())
                break;
            num = num * 10 + (s[j] - '0');
            if(num <= 255) {
                cur += s[j];
                dfs(s, j + 1, cur + '.', n + 1);
            }
            if(num == 0) break;
        }
    }
};

Source

• Restore IP Addresses - LeetCode