Question

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

Example 1:

Input: "hello" Output: "holle"

Example 2:

Input: "leetcode" Output: "leotcede"

Note:
The vowels does not include the letter "y".

Difficulty:Easy

Category:

Analyze

这道题目要求交换字符串左右的元音字母a, e, i，o，u, A，E, I，O, U, 我们只用在找到左右都是元音字母的时候进行交换，然后左指针++, '右指针--'就可以继续寻找下一个了。整体代码结构：

public:
string reverseVowels(string s) {
  int left = 0, right = s.size() - 1;
  while (left < right) {
    if (左右都是元音字母) {
      swap(s[left++], s[right--]);
    }
  }

主要需要实现的就是里面寻找到左右都是元音字母的过程。

Solution

Solution 1

class Solution {
 public:
  string reverseVowels(string s) {
    int left = 0, right = s.size() - 1;
    while (left < right) {
      if (isVowel(s[left]) && isVowel(s[right])) {
        swap(s[left++], s[right--]);
      } else if (isVowel(s[left])) {
        right--;
      } else {
        left++;
      }
    }
    return s;
  }
  bool isVowel(char c) {
    return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' || c == 'A' || c == 'E' || c == 'I' || c == 'O' ||
           c == 'U';
  }
};

Solution 2

我们尝试使用C++中String类提供的函数来更简洁的完成这个查找的过程。

find_first_of Find character in string (public member function )
find_last_of Find character in string from the end (public member function )

class Solution {
 public:
  string reverseVowels(string s) {
    int left = 0, right = s.size() - 1;
    while (left < right) {
      left = s.find_first_of("aeiouAEIOU", left);
      right = s.find_last_of("aeiouAEIOU", right);
      if (left < right) {
        swap(s[left++], s[right--]);
      }
    }
    return s;
  }
};

Solution3

第三种方式是使用String类型里面的Find函数，来自于博客：Reverse Vowels of a String 翻转字符串中的元音字母

把元音字母都存在一个字符串里，然后每遇到一个字符，就到元音字符串里去找，如果存在就说明当前字符是元音字符，参见代码如下：

class Solution {
 public:
  string reverseVowels(string s) {
    int left = 0, right = s.size() - 1;
    string t = "aeiouAEIOU";
    while (left < right) {
      if (t.find(s[left]) == string::npos)
        ++left;
      else if (t.find(s[right]) == string::npos)
        --right;
      else
        swap(s[left++], s[right--]);
    }
    return s;
  }
};

Reverse Vowels of a String