[LeetCode] 248. Strobogrammatic Number III 对称数III

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

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