[leetcode]Reverse Words in a String

Reverse Words in a String

Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

click to show clarification.

Clarification:

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string.

算法思路：

栈。（当然也可以逆序，就不需要借助栈了）

代码如下：

 1 public class Solution {
 2     public String reverseWords(String s) {
 3         if(s == null || s.trim().length() == 0) return "";
 4         Stack<String> stack = new Stack<String>();
 5         s = s.trim();
 6         int start = 0,i = 0;
 7         for(; i < s.length(); i++){
 8             if(s.charAt(i) == ' '){
 9                 String word = s.substring(start, i);
10                 stack.add(word);
11                 while(s.charAt(i) == ' ') i++;
12                 start = i;
13             }
14         }
15         stack.add(s.substring(start, i));
16         StringBuilder sb = new StringBuilder();
17         while(!stack.isEmpty()){
18             sb.append(stack.pop());
19             sb.append(' ');
20         }
21         return sb.toString().trim();
22     }
23 }

第二遍：

真真是好久没刷了，连trim方法都不会用了，囧，这一次，感觉s.substring()效率太低，试图用sb逐个append，但是想了想，感觉略蛋疼，还是一个一个截吧，这次采取的是StringBuilder中的insert方法，不再需要借助栈了。

代码如下：

public class Solution {
    public String reverseWords(String s) {
        if(s == null || s.length() == 0) return "";
        int start = 0, end = 0;
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < s.length(); i++){
            while(i < s.length() && s.charAt(i) == ' ') i++;
            start = i;
            while(i < s.length() && s.charAt(i) != ' ') i++;
            if(start == i) continue;
            sb.insert(0," ");
            sb.insert(0, s.substring(start,i));
        }
        return sb.toString().trim();
    }
}