题目描述(Medium)
Given a string containing only digits, restore it by returning all possible valid IP address combinations.
题目链接
https://leetcode.com/problems/restore-ip-addresses/description/
Example 1:
Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]
算法分析
dfs,每次将当前的位数字放入ip中(ip.size()==4),注意判断剩余字符的情况,以及前缀不可以为0。
提交代码:
class Solution {
public:
vector<string> restoreIpAddresses(string s) {
vector<string> result;
vector<string> ip;
dfs(result, ip, s, 0);
return result;
}
private:
void dfs(vector<string>& result, vector<string>& ip, string& s, int start) {
if (ip.size() == 4 && start == s.size()) {
result.push_back(ip[0] + '.' + ip[1] + '.' + ip[2] + '.' + ip[3]);
return;
}
// 如果剩余字符超过最大需要ip
if ((s.size() - start) > (4 - ip.size())*3) return;
// 如果剩余字符不足剩余ip所需
if ((s.size() - start) < (4 - ip.size())) return;
int num = 0;
for (int i = start; i < start + 3; ++i) {
num = 10 * num + s[i] - '0';
if (num > 255 || num < 0) return;
ip.push_back(s.substr(start, i -start + 1));
dfs(result, ip, s, i + 1);
ip.pop_back();
if (num == 0) break; // 不允许前缀为0,但允许单个0
}
}
};