345. 反转字符串中元音字母的位置 Reverse Vowels of a String

Write a function that takes a string as input and reverse only the vowels of a string.

Example 1:
Given s = "hello", return "holle".

Example 2:
Given s = "leetcode", return "leotcede"

题意：反转字符串中元音字母的位置

方法1：用栈保存元音字符串，时间复杂度为O(2n) 

static public string ReverseVowels(string s) {
	Stack<char> vowelsStack = new Stack<char>();
	for (int i = 0; i < s.Length; i++) {
		char c = s[i];
		if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' ||
			c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U') {
			vowelsStack.Push(c);
		}
	}
	string rsult = "";
	for (int i = 0; i < s.Length; i++) {
		char c = s[i];
		if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' ||
			c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U') {
			rsult += vowelsStack.Pop();
		} else {
			rsult += c;
		}
	}
	return rsult;
}

方法2：使用两个指针，时间复杂度为O(n/2)

public class Solution {
    public string ReverseVowels(string s) {
		char[] cArr = s.ToArray();
		int l = 0, r = s.Length - 1;
		while (l < r) {
			while (l < r && !IsVowels(cArr[l])) l++;
			while (l < r && !IsVowels(cArr[r])) r--;
			char temp = cArr[l];
			cArr[l] = cArr[r];
			cArr[r] = temp;
			l++;
			r--;
		}
		return new string(cArr);
    }
    
	public bool IsVowels(char c) {
	    return (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' || c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U');
    }
}

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