[LeetCode]Restore IP Addresses

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

For example:

Given

"25525511135"

,

return

["255.255.11.135", "255.255.111.35"]

. (Order does not matter)

DFS+剪枝

注意双0的情况

class Solution {
public:
vector<string> restoreIpAddresses(string s) {
vector<string> ret;
string ip;
dfs(s,0,0,ip,ret);
return ret;
}
void dfs(string s,size_t start,int step,string ip,vector<string> &ret){
if(start==s.size()&&step==4){
ip.resize(ip.size()-1);
ret.push_back(ip);
return;
}
if(s.size()-start>3*(4-step))
return;
if(s.size()-start<4-step)
return;
int num = 0;
for(size_t i=start; i<start+3; ++i){
ip =ip+s[i];
num = 10 * num + s[i]-'0';
if(num <= 255)
dfs(s,i+1,step+1,ip+'.',ret);
if(num == 0) //no single 0
break;
}
}
};