LeetCode 247. Strobogrammatic Number II

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

题目：

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

Find all strobogrammatic numbers that are of length = n.

For example,
Given n = 2, return ["11","69","88","96"].

题解：

base case 分奇数偶数两种. 拿到base case, 在每一个 string 前后加上"1", "1"; "8","8";...等等添加到新的res中去. 

corner case是若是string长度大于1, "0"不能加在最前面. 所以用 m != n搞定.

Time Complexity: O(5^n), exponential.

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

AC Java:

public class Solution {
    public List<String> findStrobogrammatic(int n) {
        return findHelper(n, n);
    }
    
    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;
    }
}

跟上Strobogrammatic Number III.

类似Strobogrammatic Number.