Leetcode 22. Generate Parentheses Restore IP Addresses

backtracking and invariant during generating the parathese

righjt > left  (open bracket and cloase barckst)

class Solution {
    //["((()))","(()())","(())()","()(())","()()()","())(()"] wrong case --> change right > left  the numebr of bracket is the invariant
    List<String> res = new ArrayList<>();
    public List<String> generateParenthesis(int n) {
        //back((new StringBuilder()).append('('),2*n, 1, n, n);
        back((new StringBuilder()), 2*n, 0, n, n);
        return res;
    }
    void back(StringBuilder temp, int n, int pos, int left, int right){//pos start from 1
        if(pos >= n){
            //temp.append(")"); // problem from here
            System.out.println(pos);
            res.add(temp.toString());
            return;
        }
        if(left > 0 ){
            temp.append("(");
            back(temp,n, pos+1, left-1, right);
            temp.setLength(temp.length()-1);
        }
        if(right >  left ){
            temp.append(")");
            back(temp, n, pos+1, left, right-1);
            temp.setLength(temp.length()-1);
        }
        
        
        
    }
}

Restore IP Addresses

//insert element into the string

class Solution {
    //invariant rule: each number are 
    // use the immuniateble of String
    List<String> res = new ArrayList<String>();
    public List<String> restoreIpAddresses(String s) {
        back(0, s, new String(), 0);
        return res;
    }
    
    void back(int next, String s, String str , int num){ //num: there are only three dots.
        if(num == 3){
            //if(next==s.length()) return;
            if(!valid(s.substring(next, s.length()))) return;
            res.add(str+s.substring(next, s.length()));
            return;
        }
        //for each step, move one digit or two or three
        for(int i = 1; i <=3; i++ ){
            //check string
            if(next+i > s.length()) continue;
            String sub = s.substring(next, next+i);//
            if(valid(sub)){
                back(next+i, s, str+sub+'.', num+1);
            }
        }
    }
    boolean valid(String sub){
        if(sub.length() == 0 || sub.length()>=4) return false;
        if(sub.charAt(0) == '0') {
            //System.out.println(sub.equals("0"));
            return sub.equals("0"); // not check '0' weired
        }
        int num = Integer.parseInt(sub);
        if(num >255 || num <0) return false;
        else return true;
    }
}

//core idea: move one step or 2 step or three based on the question (0 - 255) also append . and substring

use string instead stringBuilder  (immuatable)