LeetCode – Lexicographical Numbers (Java)

public List<Integer> lexicalOrder(int n) {
    int c=0;
    int t=n;
    while(t>0){
        c++;
        t=t/10;
    }
 
    ArrayList<Integer> result = new ArrayList<Integer>();
    char[] num = new char[c];
 
    helper(num, 0, n, result);
 
    return result;
}
 
public void helper(char[] num, int i, int max, ArrayList<Integer> result){
    if(i==num.length){
        int val = convert(num);
        if(val <=max)
            result.add(val);
        return;
    }
 
    if(i==0){
        for(char c='1'; c<='9'; c++){
            num[i]=c;
            helper(num, i+1, max, result);
        }
    }else{
        num[i]='a';
        helper(num, num.length, max, result);
 
        for(char c='0'; c<='9'; c++){
            num[i]=c;
            helper(num, i+1, max, result);
        }
    }
 
}
 
private int convert(char[] arr){
    int result=0;
    for(int i=0; i<arr.length; i++){
        if(arr[i]>='0'&&arr[i]<='9')
            result = result*10+arr[i]-'0';
        else
            break;
    }
    return result; 
}

public List<Integer> lexicalOrder(int n) {
    List<String> list = new ArrayList<>();
    for(int i=1;i<=n;i++){
        list.add(String.valueOf(i));
    }
 
    Collections.sort(list, new Comparator<String>(){
        public int compare(String a, String b){
            int i=0;
            while(i<a.length()&&i<b.length()){
                if(a.charAt(i)!=b.charAt(i)){
                    return a.charAt(i)-b.charAt(i);
                }
                i++;
            }
 
            if(i>=a.length()){
                return -1;
            }
 
            return 1;
        }
    });
 
    List<Integer> result = new ArrayList<>();
    for(String s: list){
        result.add(Integer.parseInt(s));
    }
 
    return result;
}