LeetCode 266. Palindrome Permutation

原题链接在这里：https://leetcode.com/problems/palindrome-permutation/

题目：

Given a string, determine if a permutation of the string could form a palindrome.

For example,
"code" -> False, "aab" -> True, "carerac" -> True.

题解：

看能否配对出现.

Time Complexity: O(n). Space: O(n).

AC Java:

public class Solution {
    public boolean canPermutePalindrome(String s) {
        if(s == null || s.length() <= 1){
            return true;
        }
        HashSet<Character> hs = new HashSet<Character>();
        for(int i = 0; i<s.length(); i++){
            char c = s.charAt(i);
            if(!hs.contains(c)){
                hs.add(c);
            }else{
                hs.remove(c);
            }
        }
        return hs.size() == 0 || hs.size() == 1;
    }
}

可以用bitMap

Time Complexity: O(n). Space: O(256).

public class Solution {
    public boolean canPermutePalindrome(String s) {
        if(s == null || s.length() <= 1){
            return true;
        }
        int [] map = new int[256];
        for(int i = 0; i<s.length(); i++){
            map[s.charAt(i)]++;
        }
        int count = 0;
        for(int i = 0; i<256; i++){
            if(count == 0 && map[i]%2 == 1){ //第一次出现frequency为奇数的char
                count++;
            }else if(map[i] % 2 == 1){ //第二次出现frequency为奇数的char
                return false;
            }
        }
        return true;
    }
}

类似Longest Palindrome. 

跟上Palindrome Permutation II.