151. Reverse Words in a String

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

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

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

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.

解法1：

用一个stack来存储每一个找出来的word，然后再一个个pop出来同时加上space
C++

1

Java

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public class Solution {
    public String reverseWords(String s) {
        if (s == null || s.length() == 0) {
            return s;
        }
        Stack<String> words = new Stack<String>();
        int i = 0, j = 0;
        while (i < s.length()) {
            while (i < s.length() && s.charAt(i) == ' ') {
                i++;
            }
            j = i + 1;
            while (j < s.length() && s.charAt(j) != ' ') {
                j++;
            }
            if (i < s.length()) {
                words.push(s.substring(i, j));
            }
            i = j;
        }
        StringBuilder builder = new StringBuilder();
        while (!words.empty()) {
            builder.append(words.pop());
            builder.append(' ');
        }
        if (builder.length() > 0) {
            builder.setLength(builder.length() - 1);        
        }
        return builder.toString();
    }
}