266. Palindrome Permutation

O(n) time O(1) space

class Solution {
public:
    bool canPermutePalindrome(string s) {
        int f[128] = {0};
        int odd_cnt = 0;
        for (char c : s) {
            odd_cnt += ((++f[c] & 1) << 1) - 1; // 奇数++偶数--
        }
        return odd_cnt <= 1;
    }
};