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

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

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

public class Solution {
    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;
    }
}