246. Strobogrammatic Number

Probelm:

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

Analysis:
列举出可以rotate的数对，然后用同向双指针检查是否匹配。
Solution:

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class Solution { public boolean isStrobogrammatic(String num) { Map<Character, Character> map = new HashMap<Character, Character>(); map.put('6', '9'); map.put('9', '6'); map.put('0', '0'); map.put('1', '1'); map.put('8', '8'); int l = 0, r = num.length() - 1; while (l <= r) { char c = num.charAt(l); if (!map.containsKey(c)) return false; if (num.charAt(r) != map.get(c)) return false; l++; r--; } return true; } }