原题链接在这里：https://leetcode.com/problems/strobogrammatic-number/
题目：
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.
Example 1:
Input: "69"
Output: true
Example 2:
Input: "88"
Output: true
Example 3:
Input: "962"
Output: false
题解：
把几种对应关系加到HashMap中，两个指针向中间夹逼.
Time Complexity: O(n). Space: O(1).
AC Java:
class Solution {
public boolean isStrobogrammatic(String num) {
if(num == null || num.length() == 0){
return true;
}
HashMap<Character, Character> hm = new HashMap<>();
String cans = "0011886996";
for(int i = 0; i<cans.length(); i+=2){
hm.put(cans.charAt(i), cans.charAt(i+1));
}
int l = 0;
int r = num.length() - 1;
while(l <= r){
char left = num.charAt(l);
char right = num.charAt(r);
if(!hm.containsKey(left) || !hm.containsKey(right) || hm.get(left) != right || hm.get(right) != left){
return false;
}
l++;
r--;
}
return true;
}
}