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Restore IP Addresses

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Description

https://leetcode.com/problems/restore-ip-addresses/description/

Solution

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class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> ret;
        if (s.size() > 12 || s.size() < 4) return ret;
        int index = 0;
        vector<char> cur;
        check(ret, s, 0, cur, 0);
        return ret;
    }
    bool valid(string& s, int start, int bias) {
        if (bias > 3) return false;
        if (s[start] == '0' && bias > 1) return false;
        if (bias < 3) return true;
        int target = s[start] - '0';
        target = 10 * target + s[start + 1] - '0';
        target = 10 * target + s[start + 2] - '0';
        return (target <= 255);
    }
    
    void check(vector<string>& ret, string& s, int start, vector<char>& cur, int depth) {
        int size = s.size();
        
        if (start == size) {
            if (depth == 4) {
                string s(cur.begin(), cur.end() - 1); //Skip the last '.'
                ret.push_back(s);
            }else
                return;
        }
        
        int bias = 1;
        while (start + bias <= size && bias <= 3) {
            if ( !valid(s, start, bias) ) break;
            cur.push_back(s[start + bias - 1]);
            cur.push_back('.');
            check(ret, s, start + bias, cur, depth + 1);
            cur.pop_back();
            ++bias;
        }
        while (bias > 1) {cur.pop_back(); --bias; }
        return;
    }
};