248. Strobogrammatic Number III

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Write a function to count the total strobogrammatic numbers that exist in the range of low <= num <= high.

For example,
Given low = "50", high = "100", return 3. Because 69, 88, and 96 are three strobogrammatic numbers.

Note:
Because the range might be a large number, the low and high numbers are represented as string.

一刷
题解：DFS
创建基本pair

 private static final char[][] pairs = {{'0', '0'}, {'1', '1'}, {'6', '9'}, {'9', '6'}, {'8', '8'}};

从两端开始插入。
遍历的范围是从low.length() 到 high.length()

public class Solution {
    private static final char[][] pairs = {{'0', '0'}, {'1', '1'}, {'6', '9'}, {'9', '6'}, {'8', '8'}};
    private int count = 0;
    
    
    public int strobogrammaticInRange(String low, String high) {
        for(int len = low.length(); len<=high.length(); len++){
            char[] c = new char[len];
            dfs(low, high, c, 0, len-1);
        }
        return count;
    }
    
    public void dfs(String low, String high, char[] c, int left, int right){
        if(left>right){
            String s = new String(c);
            if((s.length() == low.length() && s.compareTo(low)<0)
               || (s.length() == high.length() && s.compareTo(high)>0)) return;
            count++;
            return;
        }
        for(char[] p:pairs){
            c[left] = p[0];
            c[right] = p[1];
            if(c.length != 1 && c[0] == '0') continue;
            if(left == right && p[0]!=p[1]) continue;
            dfs(low, high, c, left+1, right-1);
        }
    }
}