LeetCode:Restore IP Addresses

Restore IP Addresses

Total Accepted: 57183  Total Submissions: 241109  Difficulty: Medium

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)

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  Backtracking String

思路：

将字符串分成4份（使用3个指针），当4部分都合法时才valid。

c++ code:

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        
        vector<string> ret;
        int len = s.size();
        
        for(int i=1;i<4 && i<len-2;i++) { //第1个分隔点
            for(int j=i+1;j<i+4 && j<len-1;j++) { //第2个分隔点
                for(int k=j+1;k<j+4 && k<len;k++) { //第3个分隔点
                    string s1 = s.substr(0,i);
                    string s2 = s.substr(i,j-i);
                    string s3 = s.substr(j,k-j);
                    string s4 = s.substr(k);
                    if(isLegal(s1) && isLegal(s2) && isLegal(s3) && isLegal(s4)) {
                        ret.push_back(s1+"."+s2+"."+s3+"."+s4);
                    }
                }
            }
        }
        return ret;
    }
    
    // 自定义函数
    bool isLegal(string s) {
        if(s.size()>3 || s.size()==0 || (s.size()>1 && s[0]=='0') || atoi(s.c_str())>255)
            return false;
        return true;
    }
};