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.

246. Strobogrammatic Number和247. Strobogrammatic Number II的拓展，求一个区间的数里有多少个对称数。有了247一题作基础，这里我们可以先用迭代方法求出所有长度满足的数，再通过与low,high的compare比较，选出最终的结果并count。

Java:

public class Solution {

    public List<String> helper(int n, int max) {

        if (n == 0) return new ArrayList<String>(Arrays.asList(""));

        if (n == 1) return new ArrayList<String>(Arrays.asList("1", "8", "0"));

        List<String> list = helper(n - 2, max);

        List<String> result = new ArrayList<String>();

        for (int i = 0; i < list.size(); i++) {

            String s = list.get(i);

            if (n != max) result.add("0" + s + "0");

            result.add("1" + s + "1");

            result.add("8" + s + "8");

            result.add("6" + s + "9");

            result.add("9" + s + "6");

        }

        return result;

    }

    public int strobogrammaticInRange(String low, String high) {

        int count = 0;

        List<String> result = new ArrayList<String>();

        for (int i = low.length(); i <= high.length(); i++) {

            result.addAll(helper(i, i));

        }

        for (String str : result) {

            if (str.length() == low.length() && str.compareTo(low) < 0 || str.length() == high.length() && str.compareTo(high) > 0) continue;

            count++;

        }

        return count;

    }

}

Java:

public class Solution {

    private int count = 0;

    private Map<Character, Character> map = new HashMap<>();

    public int strobogrammaticInRange(String low, String high) {

        if (low == null || low.length() == 0 || high == null || high.length() == 0) {

            return 0;

        }

        fillMap();

        for (int n = low.length(); n <= high.length(); n++) {

            char[] arr = new char[n];

            getStrobogrammaticNumbers(arr, 0, n - 1, low, high);

        }

        return count;

    }

    private void getStrobogrammaticNumbers(char[] arr, int start, int end, String low, String high) {

        if (start > end) {

            String s = new String(arr);

            if ((s.length() == 1 || s.charAt(0) != '0') && compare(low, s) && compare(s, high)) {

                count++;

            }

            return;

        }

        for (char c : map.keySet()) {

            arr[start] = c;

            arr[end] = map.get(c);

            if ((start < end) || (start == end && map.get(c) == c)) {

                getStrobogrammaticNumbers(arr, start + 1, end - 1, low, high);

            }

        }

    }

    // Return true if s1 <= s2

    private boolean compare(String s1, String s2) {

        if (s1.length() == s2.length()) {

            if (s1.compareTo(s2) <= 0) {

                return true;

            } else {

                return false;

            }

        }

        return true;

    }

    private void fillMap() {

        map.put('0', '0');

        map.put('1', '1');

        map.put('8', '8');

        map.put('6', '9');

        map.put('9', '6');

    }

}

类似题目：

[LeetCode] 246. Strobogrammatic Number 对称数

[LeetCode] 247. Strobogrammatic Number II 对称数II

All LeetCode Questions List 题目汇总

巴特西

[LeetCode] 248. Strobogrammatic Number III 对称数III

All LeetCode Questions List 题目汇总

最新文章

热门文章