Refactor TruncateSentence1816 with StringBuilder and add complexity a…

src/main/java/com/leetcode/easy/TruncateSentence1816.java

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+// Tags: String
+package com.leetcode.easy;
+
+public class TruncateSentence1816 {
+    /**
+     * Truncates the sentence to only contain the first k words.
+     *
+     * <p>A sentence is a list of words that are separated by a single space with no leading or trailing spaces.
+     * This method splits the input string and reconstructs it using only the first k words.</p>
+     *
+     * <h3>Algorithm:</h3>
+     * <ol>
+     *   <li>Split the input string by spaces to get all words</li>
+     *   <li>Iterate through the first k words and append them to a StringBuilder</li>
+     *   <li>Return the concatenated result as a String</li>
+     * </ol>
+     *
+     * <h3>Time Complexity: O(n)</h3>
+     * <ul>
+     *   <li><b>split() operation:</b> O(n) - Scans the entire input string of length n once to find spaces</li>
+     *   <li><b>For loop:</b> O(k) iterations × O(1) per iteration = O(k)</li>
+     *   <li><b>StringBuilder appends:</b> O(m) total where m is total characters in first k words (amortized O(1) per append)</li>
+     *   <li><b>toString() call:</b> O(m) to create the final String</li>
+     *   <li><b>Total:</b> O(n) + O(k) + O(m) = O(n) since k ≤ n and m ≤ n</li>
+     * </ul>
+     *
+     * <h3>Space Complexity: O(n)</h3>
+     * <ul>
+     *   <li><b>words array:</b> O(n) - Stores all word references and substring objects from split</li>
+     *   <li><b>StringBuilder:</b> O(m) - Internal buffer stores up to m characters (first k words + spaces)</li>
+     *   <li><b>Result String:</b> O(m) - New String object created from StringBuilder</li>
+     *   <li><b>Total:</b> O(n) dominant, as the split array uses O(n) space regardless of k</li>
+     * </ul>
+     *
+     * <h3>Example:</h3>
+     * <pre>
+     * Input: s = "Hello how are you", k = 2
+     * Output: "Hello how"
+     * Explanation: Returns only the first 2 words separated by a space
+     * </pre>
+     *
+     * @param s the input sentence consisting of words separated by single spaces (length ≥ 1)
+     * @param k the number of words to keep from the beginning (1 ≤ k ≤ number of words in s)
+     * @return a new string containing only the first k words separated by spaces
+     */
+    public String truncateSentence(String s, int k) {
+        String[] words = s.split(" ");
+        StringBuilder res = new StringBuilder();
+        for (int i = 0; i < k; i++) {
+            if (i > 0) res.append(" ");
+            res.append(words[i]);
+        }
+        return res.toString();
+    }
+}

src/test/java/com/leetcode/easy/TruncateSentence1816Test.java

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+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class TruncateSentence1816Test {
+    private TruncateSentence1816 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new TruncateSentence1816();
+    }
+
+    @Test
+    void testTruncateSentence_Example1() {
+        String s = "Hello how are you Contestant";
+        int k = 4;
+
+        String result = solution.truncateSentence(s, k);
+
+        assertEquals("Hello how are you", result);
+    }
+
+    @Test
+    void testTruncateSentence_Example2() {
+        String s = "What is the solution to this problem";
+        int k = 4;
+
+        String result = solution.truncateSentence(s, k);
+
+        assertEquals("What is the solution", result);
+    }
+
+    @Test
+    void testTruncateSentence_Example3() {
+        String s = "chopper is not a tanuki";
+        int k = 5;
+
+        String result = solution.truncateSentence(s, k);
+
+        assertEquals("chopper is not a tanuki", result);
+    }
+}