LeetCode 246. Strobogrammatic Number

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;
    }
}

跟上Strobogrammatic Number II, Strobogrammatic Number III.