345. Reverse Vowels of a String

题目：

Given a string s, reverse only all the vowels in the string and return it.

The vowels are 'a', 'e', 'i', 'o', and 'u', and they can appear in both cases.

Example 1:

Input: s = "hello"
Output: "holle"

Example 2:

Input: s = "leetcode"
Output: "leotcede"

Constraints:

1 <= s.length <= 3 * 105
s consist of printable ASCII characters.

思路：

要交换位置，明显的双指针。先用哈希set记录下元音，可以只记录全大写或者全小写，在判断的时候多写一些语句也行，不过这里偷懒就把大小写元音都记录下来了。两个指针分别index = 0 和 n - 1，只要左指针的当前字母不在哈希set内，右移；同理右指针的当前字母不在哈希set内就左移，如果当前左指针index小于右指针index，则交换元素并且再次移动指针。最后返回字符串即可。

代码：

class Solution {
public:
string reverseVowels(string s) {
unordered_set<char> mp= {'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'};
int n = s.size(), i = 0, j = n - 1;
while (i < j) {
while (i < n && !mp.count(s[i]))
i++;
while ( j > 0 && !mp.count(s[j]))
j--;
if (i < j) {
swap(s[i], s[j]);
i++;
j--;
}
}
return s;
}
};

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