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.

Related to confusing number

O(N) time and O(N) space

 1 public class Solution {
 2     public boolean isStrobogrammatic(String num) {
 3         if (num==null || num.length()==0) return false;
 4         StringBuffer buf = new StringBuffer();
 5         for (int i=0; i<num.length(); i++) {
 6             char c = num.charAt(i);
 7             if (c!='0' && c!='1' && c!='6' && c!= '8' && c!= '9') return false;
 8             else if (c=='0' || c=='1' || c=='8') buf.insert(0, c);
 9             else if (c == '6') buf.insert(0, '9');
10             else if (c == '9') buf.insert(0, '6');
11         }
12         return buf.toString().equals(num);
13     }
14 }

O(N) time and O(1) space, use two pointers

 1 public class Solution {
 2     public boolean isStrobogrammatic(String num) {
 3         HashMap<Character, Character> map = new HashMap<Character, Character>();
 4         map.put('1','1');
 5         map.put('0','0');
 6         map.put('6','9');
 7         map.put('9','6');
 8         map.put('8','8');
 9         int left = 0, right = num.length() - 1;
10         while(left <= right){
11             // 如果字母不存在映射或映射不对，则返回假
12             if(!map.containsKey(num.charAt(right)) || num.charAt(left) != map.get(num.charAt(right))){
13                 return false;
14             }
15             left++;
16             right--;
17         }
18         return true;
19     }
20 }