93. Restore IP Addresses

https://leetcode.com/problems/restore-ip-addresses/discuss/30944/Very-simple-DFS-solution

 

class Solution {
    List<String> res = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        backtrack(s, "", 0, 0);
        return res;
        
    }
    
    public void backtrack(String s, String str, int index, int count) {
        if(count > 4) return;
        if(count == 4 && index == s.length()) res.add(str);
        
        for(int i = 1; i < 4; i++) {
            if(index + i > s.length()) break;
            String sub = s.substring(index, index + i);
            if((sub.startsWith("0") && sub.length() > 1) || (Integer.parseInt(sub) > 255)) break;
            backtrack(s, str + sub + (count == 3?"" : "."), index + i, count+1);   //因为是直接str+什么什么 所以不改变str本身的值
            
        }
    }
}