LeetCode 248. Strobogrammatic Number III

原题链接在这里：https://leetcode.com/problems/strobogrammatic-number-iii/

题目：

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.

题解：

类似Strobogrammatic Number II. 从low.length()到high.length()长度范围内用Strobogrammatic Number II得到的所有结果算范围内个数.

Time Complexity: O(len * 5^(len/2) * 5^(len/2)), len = high.length(), 第一个5^(len/2)是findHelper用时, 第二个5^(len/2)是iterate当前ls用时.

Space: O(len/2  + 5^(len/2)), len/2层stack, 5^(len/2)是当前base的大小.

AC Java:

public class Solution {
    public int strobogrammaticInRange(String low, String high) {
        int count = 0;
        for(int len = low.length(); len<=high.length(); len++){
            List<String> ls = findHelper(len, len);
            for(String s : ls){
                if((s.length() == low.length() && s.compareTo(low) < 0) || (s.length() == high.length() && s.compareTo(high) > 0)){
                    continue;
                }
                count++;
            }
        }
        return count;
    }
    
    private List<String> findHelper(int cur, int max){
        if(cur == 0){
            return new ArrayList<String>(Arrays.asList(""));
        }
        if(cur == 1){
            return new ArrayList<String>(Arrays.asList("0", "1", "8"));
        }
        
        List<String> res = new ArrayList<String>();
        List<String> base = findHelper(cur-2, max);
        for(String s : base){
            if(cur != max){
                res.add("0" + s + "0");
            }
            res.add("1" + s + "1");
            res.add("8" + s + "8");
            res.add("6" + s + "9");
            res.add("9" + s + "6");
        }
        return res;
    }
}

类似Confusing Number II.