345. Reverse Vowels of a String Java Solutions

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".

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public class Solution {
    public String reverseVowels(String s) {
        if(null == s) return s;
        char[] tmp = s.toCharArray();
        Stack<Character> stack = new Stack<Character>();
        for(int i = 0; i<tmp.length; i++){
            if(isVowels(tmp[i])){
                     stack.push(tmp[i]);
                }
        }
        for(int i = 0; i<tmp.length; i++){
            if(isVowels(tmp[i])){
                    tmp[i] = stack.pop();
                }
        }
        String res = new String(tmp);
        return res;
    }
    
    public boolean isVowels(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;
    }