LeetCode 1816. Truncate Sentence【字符串】简单

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A sentence is a list of words that are separated by a single space with no leading or trailing spaces. Each of the words consists of only uppercase and lowercase English letters (no punctuation).

• For example, "Hello World", "HELLO", and "hello world hello world" are all sentences.

You are given a sentence s​​​​​​ and an integer k​​​​​​. You want to truncate s​​​​​​ such that it contains only the first k​​​​​​ words. Return s​​​​​​ after truncating it.

Example 1:

Input: s = "Hello how are you Contestant", k = 4
Output: "Hello how are you"
Explanation:
The words in s are ["Hello", "how" "are", "you", "Contestant"].
The first 4 words are ["Hello", "how", "are", "you"].
Hence, you should return "Hello how are you".

Example 2:

Input: s = "What is the solution to this problem", k = 4
Output: "What is the solution"
Explanation:
The words in s are ["What", "is" "the", "solution", "to", "this", "problem"].
The first 4 words are ["What", "is", "the", "solution"].
Hence, you should return "What is the solution".

Example 3:

Input: s = "chopper is not a tanuki", k = 5
Output: "chopper is not a tanuki"

Constraints:

• 1 <= s.length <= 500
• k is in the range [1, the number of words in s].
• s consist of only lowercase and uppercase English letters and spaces.
• The words in s are separated by a single space.
• There are no leading or trailing spaces.

题意：句子 是一个单词列表，列表中的单词之间用单个空格隔开，且不存在前导或尾随空格。每个单词仅由大小写英文字母组成（不含标点符号）。

给你一个句子 s​​​​​​ 和一个整数 k​​​​​​ ，请你将 s​​ 截断 ​，​​​使截断后的句子仅含 前 k​​​​​​ 个单词。返回 截断 s​​​​​​ 后得到的句子。

---

解法 字符串+模拟

根据题意模拟，一边拼接答案的同时，一边对空格进行计数即可。

//Java version
class Solution {
    public String truncateSentence(String s, int k) {
        StringBuilder sb = new StringBuilder();
        int cnt = 0;
        for (int i = 0, n = s.length(); i < n && cnt < k; ++i) {
            char c = s.charAt(i);
            if (c == ' ') ++cnt;
            if (cnt < k) sb.append(c);
        }
        return sb.toString();
    }
}
//执行用时：1 ms, 在所有 Java 提交中击败了65.28% 的用户
//内存消耗：36.5 MB, 在所有 Java 提交中击败了80.68% 的用户

也可以使用下标 i 记录答案的结束位置：

//C++ version
class Solution {
public:
    string truncateSentence(string s, int k) {
        int i = 0, n = s.size();
        while (i < n) {
            if (s[i] == ' ' && --k == 0) break;
            ++i;
        }
        return s.substr(0, i);
    }
};
//执行用时：0 ms, 在所有 C++ 提交中击败了100.00% 的用户
//内存消耗：6.4 MB, 在所有 C++ 提交中击败了34.21% 的用户

1816. 截断句子