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.

思路：建立一个mapping的连接，用array足够，不需要hashmap，然后双指针从前往后，从后往前扫，其中-1，则false，如果互为颠倒，false。

public class Solution {
public boolean isStrobogrammatic(String num) {
if(num == null || num.length() ==0) return false;
char[] chars = num.toCharArray();
int[] map = new int[]{0,1,-1,-1,-1,-1,9,-1,8,6};

int i=0; int j=chars.length-1;
while(i<=j){
if(map[chars[i]-'0'] == -1 || map[chars[j]-'0'] == -1){
return false;
}
if(map[chars[i]-'0'] != (chars[j]-'0')) {
return false;
}
i++;
j--;
}
return true;
}
}