151.颠倒字符串中的单词，双指针，双端队列

Author: linwt

leetcode_Java/DoneTitle.txt

@@ -63,6 +63,7 @@
 145. 二叉树的后序遍历
 146. LRU 缓存
 148. 排序链表
+151. 颠倒字符串中的单词
 152. 乘积最大子数组
 155. 最小栈
 160. 相交链表

leetcode_Java/Solution0151.java

@@ -0,0 +1,87 @@
+// 151. 颠倒字符串中的单词
+
+
+/*
+1、字符串去掉头尾空格，按空格分隔字符串，倒序拼接单词
+2、拼接单词字符串可以使用StringBuilder直接构造，也可以先把单词加到列表最后拼接
+ */
+class Solution {
+    public String reverseWords(String s) {
+        String[] splits = s.trim().split(" ");
+        StringBuilder res = new StringBuilder();
+        for (int i = splits.length - 1; i >= 0; i--) {
+            String word = splits[i];
+            if (word == "") {
+                continue;
+            }
+            res.append(word);
+            if (i > 0) {
+                res.append(" ");
+            }
+        }
+        return res.toString();
+    }
+}
+
+
+/*
+去掉头尾空格，正则表达式\\s+ 连续空白字符作为分隔符，得到单词列表，列表反转，再通过空格字符连接成字符串
+ */
+class Solution {
+    public String reverseWords(String s) {
+        s = s.trim();
+        List<String> list = Arrays.asList(s.split("\\s+"));
+        Collections.reverse(list);
+        return String.join(" ", list);
+    }
+}
+
+
+/*
+双指针：从右到左，跳过空格，遍历得到一个单词的索引区间，将单词加入列表，最后列表通过空格字符连接成字符串
+ */
+class Solution {
+    public String reverseWords(String s) {
+        s = s.trim();
+        int n = s.length();
+        int left = n - 1, right = n - 1;
+        List<String> list = new ArrayList<>();
+        while (left >= 0) {
+            while (left >= 0 && s.charAt(left) != ' ') {
+                left--;
+            }
+            list.add(s.substring(left + 1, right + 1));
+            while (left >= 0 && s.charAt(left) == ' ') {
+                left--;
+            }
+            right = left;
+        }
+        return String.join(" ", list);
+    }
+}
+
+
+/*
+双端队列：从左到右，遍历字符构造出一个单词，遇到空格时且单词不为空就将单词加到队列头部，最后队列通过空格字符连接成字符串
+ */
+class Solution {
+    public String reverseWords(String s) {
+        s = s.trim();
+        s += " ";
+        int n = s.length();
+        int left = 0, right = n - 1;
+        Deque<String> queue = new ArrayDeque<>();
+        StringBuilder word = new StringBuilder();
+        while (left <= right) {
+            char c = s.charAt(left);
+            if (word.length() != 0 && c == ' ') {
+                queue.offerFirst(word.toString());
+                word.setLength(0);
+            } else if (c != ' ') {
+                word.append(s.charAt(left));
+            }
+            left++;
+        }
+        return String.join(" ", queue);
+    }
+}