LeetCode 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(2^n). Space: O(n).

AC Java:

 1 public class Solution {
 2     public List<String> findStrobogrammatic(int n) {
 3         return findHelper(n,n);
 4     }
 5     
 6     private List<String> findHelper(int m, int n){
 7         if(m == 0){
 8             return new ArrayList<String>(Arrays.asList(""));
 9         }
10         if(m == 1){
11             return new ArrayList<String>(Arrays.asList("0", "1", "8"));
12         }
13         List<String> base = findHelper(m-2, n);
14         List<String> res = new ArrayList<String>();
15         for(String s : base){
16             if(m != n){
17                 res.add("0" + s + "0");
18             }
19             res.add("1" + s + "1");
20             res.add("8" + s + "8");
21             res.add("6" + s + "9");
22             res.add("9" + s + "6");
23         }
24         return res;
25     }
26 }

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