[LeetCode] 1704. Determine if String Halves Are Alike

You are given a string s of even length. Split this string into two halves of equal lengths, and let a be the first half and b be the second half.

Two strings are alike if they have the same number of vowels ('a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'). Notice that s contains uppercase and lowercase letters.

Return true if a and b are alike. Otherwise, return false.

Example 1:
Input: s = "book"
Output: true
Explanation: a = "bo" and b = "ok". a has 1 vowel and b has 1 vowel. Therefore, they are alike.

Example 2:
Input: s = "textbook"
Output: false
Explanation: a = "text" and b = "book". a has 1 vowel whereas b has 2. Therefore, they are not alike.
Notice that the vowel o is counted twice.

Constraints:
2 <= s.length <= 1000
s.length is even.
s consists of uppercase and lowercase letters.

判断字符串的两半是否相似。

给你一个偶数长度的字符串 s 。将其拆分成长度相同的两半，前一半为 a ，后一半为 b 。
两个字符串 相似 的前提是它们都含有相同数目的元音（'a'，'e'，'i'，'o'，'u'，'A'，'E'，'I'，'O'，'U'）。注意，s 可能同时含有大写和小写字母。
如果 a 和 b 相似，返回 true ；否则，返回 false 。
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/determine-if-string-halves-are-alike
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思路

思路是 two pointer。用两个指针，从 input 字符串的左右两边往中间逼近，逼近的时候分别检查左指针和右指针各自统计了多少元音字母。最后判断的是左右两边的元音字母的个数是否相等。

复杂度

时间O(n)
空间O(1)

代码

Java实现

class Solution {
    public boolean halvesAreAlike(String s) {
        int left = 0;
        int right = 0;
        int start = 0;
        int end = s.length() - 1;
        while (start < end) {
            if (isVowel(s.charAt(start))) {
                left++;
            }
            start++;
            if (isVowel(s.charAt(end))) {
                right++;
            }
            end--;
        }
        return left == right;
    }

    private boolean isVowel(char c) {
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' || c == 'A' || c == 'E' || c == 'I' || c == 'O'
                || c == 'U') {
            return true;
        }
        return false;
    }
}