Leetcode Strobogrammatic Number

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Write a function to determine if a number is strobogrammatic. The number is represented as a string.

For example, the numbers "69", "88", and "818" are all strobogrammatic.


解题思路:

立即想到双指针,一个从左到右,一个从右到左,接下来想到哪些数满足 looks the same when rotated 180 degrees, 那就是0,1,8,(6,9),(9,6)。

我第一种方法没用Map, 纯粹比较,慢一些;第二种方法将满足要求的值存到map里。每次检查一下即可,速度会快很多。

推荐使用方法二!使用HashMap


Java code:

方法一:

public boolean isStrobogrammatic(String num) {
        if(num.length() == 0) {
            return true;
        }
        int len = num.length();
        int i = 0, j = len-1;
        boolean flag = false;
        while(i <= j) {
            char x = num.charAt(i);
            char y = num.charAt(j);
            if((x == '1' && y == '1') || (x == '8' && y == '8') || (x == '0' && y == '0')
                || (x == '6' && y == '9') || (x == '9' && y == '6')){
                flag = true;
            }else {
                flag = false;
                break;
            }
            i++;
            j--;
        }
        return flag;
    }

方法二: HashMap

 public boolean isStrobogrammatic(String num) {
        Map<Character, Character> map = new HashMap<Character, Character>();
        map.put('1', '1');
        map.put('0', '0');
        map.put('8', '8');
        map.put('6', '9');
        map.put('9', '6');
        int l = 0, r = num.length()-1;
        while(l <= r) {
            if(!map.containsKey(num.charAt(l))) {
                return false;
            }
            if(map.get(num.charAt(l)) != num.charAt(r)){
                return false;
            }
            l++;
            r--;
        }
        return true;
    }

Reference:

1. https://leetcode.com/discuss/52273/accepted-java-solution

 

posted @ 2015-10-06 07:44  茜茜的技术空间  阅读(228)  评论(0编辑  收藏  举报