LeetCode Strobogrammatic Number III

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mob604756fc5b03 2017-01-11 14:03:00

文章标签 LeetCode Recursion java ide JAVA 文章分类 Java 后端开发

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:

 1 public class Solution {
 2     public int strobogrammaticInRange(String low, String high) {
 3         int count = 0;
 4         for(int len = low.length(); len<=high.length(); len++){
 5             List<String> ls = findHelper(len, len);
 6             for(String s : ls){
 7                 if((s.length() == low.length() && s.compareTo(low) < 0) || (s.length() == high.length() && s.compareTo(high) > 0)){
 8                     continue;
 9                 }
10                 count++;
11             }
12         }
13         return count;
14     }
15     
16     private List<String> findHelper(int cur, int max){
17         if(cur == 0){
18             return new ArrayList<String>(Arrays.asList(""));
19         }
20         if(cur == 1){
21             return new ArrayList<String>(Arrays.asList("0", "1", "8"));
22         }
23         
24         List<String> res = new ArrayList<String>();
25         List<String> base = findHelper(cur-2, max);
26         for(String s : base){
27             if(cur != max){
28                 res.add("0" + s + "0");
29             }
30             res.add("1" + s + "1");
31             res.add("8" + s + "8");
32             res.add("6" + s + "9");
33             res.add("9" + s + "6");
34         }
35         return res;
36     }
37 }