Reverse String I、II、III（tag：String）

Reverse String I：

Write a function that takes a string as input and returns the string reversed.

Example：
Given s = "hello", return "olleh".

Solution1（最开始自己的思路，效率较低）：

public static String reverseString(String s) {        StringBuilder res=new StringBuilder("");        for(int i=s.length()-1;i>=0;i--)        {        res=res.append(s.charAt(i));                }        return res.toString();}

Solution2（愚蠢的我啊才知道StringBuilder人家有reverse()函数，当然效率还是较低）：

public static String reverseString(String s) {      StringBuilder sb = new StringBuilder(s);      return sb.reverse().toString();}

Solution3（这才是正解！效率即正义！）：

public static String reverseString(String s) {      char[] word = s.toCharArray();      int i = 0;      int j = s.length() - 1;      while (i < j)       {          char temp = word[i];          word[i] = word[j];          word[j] = temp;          i++;          j--;      }      return new String(word);}

Reverse String II：

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

Example：

Input: s = "abcdefg", k = 2Output: "bacdfeg"

Restrictions：

1. The string consists of lower English letters only.
2. Length of the given string and k will in the range [1, 10000]

Solution（最开始自己的思路，效率较高，就是细节条件较多，注意考虑周全）：

public String reverseStr(String s, int k) {        char res[]=s.toCharArray();        if(k>s.length())        return new String(reverseAll(res,0,s.length()-1));        for(int p=0,q=k-1;p<s.length();p=p+2*k,q=q+2*k)        {        int i=p;        int j=q<s.length()?q:s.length()-1;        reverseAll(res,i,j);        }        return new String(res);}public static char[] reverseAll(char res[],int i,int j){while(i<j)    {    char temp=res[i];    res[i]=res[j];    res[j]=temp;    i++;    j--;    }return res;}

Reverse String III：

Given a string, you need to reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.

Example 1:

Input: "Let's take LeetCode contest"Output: "s'teL ekat edoCteeL tsetnoc"

Note: In the string, each word is separated by single space and there will not be any extra space in the string.

Solution1（最开始自己的思路，效率一般，做复杂了= =）：

public String reverseWords(String s) {        String sArray[]=s.split("\\ ");        StringBuilder res=new StringBuilder("");        for(int i=0;i<sArray.length;i++)        {        res.append(String.valueOf(reverseAll(sArray[i].toCharArray(),0,sArray[i].length()-1)));        if(i!=sArray.length-1)        res.append(" ");                }        return new String(res); }public static char[] reverseAll(char res[],int i,int j){while(i<j)    {    char temp=res[i];    res[i]=res[j];    res[j]=temp;    i++;    j--;    }return res;}}

Solution2（万变不离其宗，easier one）：

public String reverseWords(String s) {      char[] s1 = s.toCharArray();      int i = 0;      for(int j = 0; j < s1.length; j++)      {          if(s1[j] == ' ')          {              reverseAll(s1, i, j - 1);              i = j + 1;          }      }      reverseAll(s1, i, s1.length - 1);      return new String(s1);}public char[] reverseAll(char res[],int i,int j){while(i<j)    {    char temp=res[i];    res[i]=res[j];    res[j]=temp;    i++;    j--;    }return res;}

总结：掌握首尾交换的中心思想。