248. Strobogrammatic Number III

和strobogrammatic II一个做法。 算出range是几位数到几位数，比如50-100是2-3位，generate所有2和3位的所有可能，看看是不是在50-100之内，是的话res++...

public class Solution {
    int res = 0;
    public int strobogrammaticInRange(String low, String high) {
        char[] ch1 = {'1','8','0','6','9'};
        char[] ch2 = {'1','8','0','9','6'};
        
        for (int i = low.length(); i<= high.length(); i++) {
            char[] s = new char[i];
            dfs(ch1, ch2, s, low, high, 0);
        }
        return res;
    }
    
    public void dfs(char[] ch1, char[] ch2, char[] s, String low, String high, int l) {

        int r = s.length - 1 - l;
        if (l > r) {
            if (Long.valueOf(String.valueOf(s)) <= Long.valueOf(high) &&
                Long.valueOf(String.valueOf(s)) >= Long.valueOf(low)) {
                    res++;
                }
        } else if (r == l) {
            for (int i = 0; i < 3; i++) {
                s[l] = ch1[i];
                dfs(ch1, ch2, s, low, high, l + 1);
            }
        } else {
            for (int i = 0; i < 5; i++) {
                if (i == 2 && l == 0) continue;
                s[l] = ch1[i];
                s[r] = ch2[i];
                dfs(ch1, ch2, s, low, high, l + 1);
            }
        }
    }
}