# 248.Strobogrammatic-Number-III

## 248. Strobogrammatic Number III

## 题目地址

<https://leetcode.com/problems/strobogrammatic-number-iii/>

## 题目描述

```
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.

Example:
Input: low = "50", high = "100"
Output: 3 
Explanation: 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.
```

## 代码

### Approach #1

```java
class Solution {
  public int strobogrammaticInRange(String low, String high) {
    List<String> list = new ArrayList<>();
    for (int l = low.length(); l <= high.length(); l++) {
      list.addAll(findStrobogrammatic(l));
    }

    int count = 0;
    for (String s: list) {
      if ((s.length() == low.length() && s.compareTo(low) < 0)
         || (s.length() == high.length() && s.compare(high) > 0)) {
        continue;
      }
      ++count;
    }
    return count;
  }

  private List<String> findStrobogrammatic(int len) {
    List<String> list = len % 2 == 0 ? 
      Arrays.asList("") : Arrays.asList("0", "1", "8");

    for (int i = len % 2 == 0 ? 0 : 1; i < len; i += 2) {
      List<String> newList = new ArrayList<>();
      for (String s: list) {
        if (i + 2 < len) {
          newList.add("0" + s + "0");
        }

        newList.add("1" + s + "1");
        newList.add("6" + s + "9");
        newList.add("8" + s + "8");
        newList.add("9" + s + "6");
      }

      list = newList;
    }
    return list;
  }
}
```

### Approach #2

Time: O(1) && Space: O(1)

```java
class Solution {
  private static final char[][] pairs = {{'0', '0'}, {'1', '1'}, {'6', '9'}, {'8', '8'}, {'9', '6'}};
  public int strobogrammaticInRange(String low, String high) {
        int[] count = {0};
    for (int len = low.length(); len <= high.length(); len++) {
      char[] c = new char[len];
      dfs(low, high, c, 0, len - 1, count);
    }
    return count[0];
  }

  public void dfs(String low, String high, char[] c, int left, int right, int[] count) {
    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[0]++;
      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, count);
    }
  }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://wentao-shao.gitbook.io/leetcode/graph-search/248.strobogrammatic-number-iii.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.