LeetCode_ String_ Medium_ 151. Reverse the words in the string

1. subject

Give you a string s , Reverse the order of words in a string .

A word is a string of non whitespace characters .s Use in At least one space Separate the words in the string .

Return words in reverse order and use Single space Connected result string .

Be careful ： Input string s There may be leading spaces in 、 Trailing spaces or multiple spaces between words . In the returned result string , Words should be separated by only a single space , And does not contain any additional spaces .

Example 1：
Input ：s = “the sky is blue”
Output ：“blue is sky the”

Example 2：
Input ：s = " hello world "
Output ：“world hello”
explain ： The inverted string cannot contain leading and trailing spaces .

Example 3：
Input ：s = “a good example”
Output ：“example good a”
explain ： If there are extra spaces between two words , The inverted string needs to reduce the space between words to only one .

Tips ：
1 <= s.length <= 10⁴
s Contains English upper and lower case letters 、 Numbers and spaces ’ ’
s At least one word exists in

Advanced ： If the string is a variable data type in the programming language you use , Please try using O(1) Additional space complexity In situ solution .

source ： Power button （LeetCode）
link ：https://leetcode.cn/problems/reverse-words-in-a-string

2. Ideas

（1） call API

（2） Manual implementation

3. Code implementation （Java）

// Ideas 1———— call  API
class Solution {
    
    public String reverseWords(String s) {
    
        // Remove the spaces at the beginning and end 
        s = s.trim();
        // Regular matching continuous white space characters as separators to segment 
        List<String> words = Arrays.asList(s.split("\\s+"));
        // Flip  words  The words in 
        Collections.reverse(words);
        // Using spaces will  words  The words in are spliced into a string and returned 
        return String.join(" ", words);
    }
}

// Ideas 2———— Manual implementation 
class Solution {
    
    public String reverseWords(String s) {
    
        // Start index and end index of each word 
        int start, end;
        StringBuilder builder = new StringBuilder();
        for (int i = s.length() - 1; i >= 0; i--) {
    
            // If you encounter spaces, skip 
            if (s.charAt(i) == ' ') {
    
                continue;
            }
            // End index found 
            end = i + 1;
            while (i >= 0 && s.charAt(i) != ' ') {
    
                i--;
            }
            // Find the start index 
            start = i + 1;
            // Intercept the current word and splice it to  builder  in 
            builder.append(s.substring(start, end));
            builder.append(' ');
        }
        // Delete the last extra space 
        builder.deleteCharAt(builder.length() - 1);
        return builder.toString();
    }
}