151. Reverse Words in a String (M)

Reverse Words in a String (M)

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

Example 1:

Input: "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: "  hello world!  "
Output: "world! hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Note:

• A word is defined as a sequence of non-space characters.
• Input string may contain leading or trailing spaces. However, your reversed string should not contain leading or trailing spaces.
• You need to reduce multiple spaces between two words to a single space in the reversed string.

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题意

将给定字符串中的非空格单词的顺序逆置，要求新字符串中前后无空白字符，且每个单词之间有一个空格。

思路

将字符串中所有单词依次存入栈中，再出栈构建新字符串即可。

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代码实现

class Solution {
    public String reverseWords(String s) {
        s = s.trim();
        Deque<String> stack = new ArrayDeque<>();
        StringBuilder sb = new StringBuilder();

        int first = 0, last = 0;
        while (last < s.length()) {
            while (last < s.length() && s.charAt(last) != ' ') {
                last++;
            }
            stack.push(s.substring(first, last));
            while (last < s.length() && s.charAt(last) == ' ') {
                last++;
            }
            first = last;
        }

        for (String word : stack) {
            if (sb.length() == 0) {
                sb.append(word);
            } else {
                sb.append(" " + word);
            }
        }

        return sb.toString();
    }
}