Strobogrammatic Number I 对称数(Easy)
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.
双指针法+HashMap
翻转后对称的数就那么几个，我们可以根据这个建立一个映射关系：8->8, 0->0, 1->1, 6->9, 9->6，然后从两边向中间检查对应位置的两个字母是否有映射关系就行了。比如619，先判断6和9是有映射的，然后1和自己又是映射，所以是对称数。
Java代码：
public boolean isStrobogrammatic(String num) {
HashMap<Character, Character> map = new HashMap<Character, Character>();
map.put('1','1');
map.put('0','0');
map.put('6','9');
map.put('9','6');
map.put('8','8');
int left = 0, right = num.length() - 1;
while(left <= right){
// 如果字母不存在映射或映射不对，则返回假
if(!map.containsKey(num.charAt(right)) || num.charAt(left) != map.get(num.charAt(right))){
return false;
}
left++;
right--;
}
return true;
}