String inversion exercises 344, 541, 557, 151

151. Flip the words in the string

Given a string , Flip each word in the string one by one .
Example 1：
Input : “the sky is blue”
Output : “blue is sky the”
Example 2：
Input : " hello world! "
Output : “world! hello”
explain : The input string can contain extra spaces before or after , But the reversed characters cannot include .
Example 3：
Input : “a good example”
Output : “example good a”
explain : If there are extra spaces between two words , Reduce the space between inverted words to just one .

explain ：
No space characters make up a word .
The input string can contain extra spaces before or after , But the reversed characters cannot include .
If there are extra spaces between two words , Reduce the space between inverted words to just one .
Answer key ：
Double pointer , Reverse traversal , Add each word to res, Finally, it is spliced into a string
sloution：

 class Solution {
    
    public String reverseWords(String s) {
    
    s=s.trim();
    int j=s.length()-1,i=j;
    StringBuilder res=new StringBuilder();
    while(i>=0){
    
        while(i >= 0 && s.charAt(i) != ' ') i--; //  Search for the first space 
            res.append(s.substring(i + 1, j + 1) + " "); //  Add words 
            while(i >= 0 && s.charAt(i) == ' ') i--; //  Skip spaces between words 
            j = i; // j  The last character pointing to the next word 
    }
     return res.toString().trim(); //  Convert to a string and return 
    }
}

541. Reverse string II

Given a string s And an integer k, You need to check every 2k Before characters k Characters to reverse .
If the remaining characters are less than k individual , Reverse all remaining characters .
If the remaining characters are less than 2k But greater than or equal to k individual , Before reversal k Characters , The rest of the characters remain the same .

Example :
Input : s = “abcdefg”, k = 2
Output : “bacdfeg”
Tips ：
The string contains only lowercase letters .
The length and k stay [1, 10000] Within the scope of .
Answer key ：
1） Double pointer Be careful String Objects are immutable objects , have access to toCharArray()
2k For a group , front k Elements are inverted , after k The first element remains unchanged .
sloution：

class Solution {
    
    public String reverseStr(String s, int k) {
    
    char[] ch = s.toCharArray();
    for(int i=0;i<ch.length;i+=2*k){
    
    int left=i;
    int right=(i+k-1<ch.length)?i+k-1:ch.length-1;// Transboundary 
      while (left <= right){
    
                char temp = ch[left];
                ch[left] = ch[right];
                ch[right] = temp;
                left++;
                right--;
            }
    }
        String str = new String(ch);
        return str;
    }
}

344. Reverse string

Write a function , Its function is to invert the input string . Input string as character array char[] Given in the form of .

Do not allocate extra space to another array , You have to modify the input array in place 、 Use O(1) To solve this problem .

You can assume that all characters in an array are ASCII Printable characters in code table .

Example 1：

Input ：[“h”,“e”,“l”,“l”,“o”]
Output ：[“o”,“l”,“l”,“e”,“h”]
Example 2：

Input ：[“H”,“a”,“n”,“n”,“a”,“h”]
Output ：[“h”,“a”,“n”,“n”,“a”,“H”]
Answer key ： Double pointer

sloution：

class Solution {
    
    public void reverseString(char[] s) {
    
    int left=0;
    int right=s.length-1;
    while(left<right){
    
        char tmp=s[left];
        s[left]=s[right];
        s[right]=tmp;
        left++;
        right--;
    }
  }
}

557. Invert the words in the string III

Given a string , You need to reverse the character order of each word in the string , Keep the initial order of spaces and words .
Example ：
Input ：“Let’s take LeetCode contest”
Output ：“s’teL ekat edoCteeL tsetnoc”

class Solution {
    
    public String reverseWords(String s) {
    
    String[] strs=s.split(" ");
    StringBuffer buffer = new StringBuffer();
    for(int i=0;i<strs.length;i++){
    
        buffer.append(new StringBuffer(strs[i]).reverse().toString());
        buffer.append(" ");
    }
    return buffer.toString().trim();
    }
}

ToString() Is the way to convert to a string Trim() It's the way to get rid of the spaces on both sides

Java StringBuffer and StringBuilder class
When you modify a string , Need to use StringBuffer and StringBuilder class .
and String Class is different from ,StringBuffer and StringBuilder Class can be modified many times , And don't generate new unused objects .

StringBuilder Class in Java 5 It was proposed that , It and StringBuffer The biggest difference between them is StringBuilder Is not thread safe （ Can't sync access ）.

because StringBuilder Compare with StringBuffer Speed advantage , So in most cases, it is recommended to use StringBuilder class . However, when the application requires thread safety , Must be used StringBuffer class .
ToString() Is the way to convert to a string Trim() It's the way to get rid of the spaces on both sides