LeetCode #1816 — EASY

Truncate Sentence

Build confidence with an intuition-first walkthrough focused on array fundamentals.

The Problem

Problem Statement

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.

Step 01

Brute Force Baseline

Problem summary: 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.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array

Example 1

"Hello how are you Contestant"
4

Example 2

"What is the solution to this problem"
4

Example 3

"chopper is not a tanuki"
5

Step 02

Core Insight

What unlocks the optimal approach

It's easier to solve this problem on an array of strings so parse the string to an array of words
After return the first k words as a sentence

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #1816: Truncate Sentence
class Solution {
    public String truncateSentence(String s, int k) {
        for (int i = 0; i < s.length(); ++i) {
            if (s.charAt(i) == ' ' && (--k) == 0) {
                return s.substring(0, i);
            }
        }
        return s;
    }
}

// Accepted solution for LeetCode #1816: Truncate Sentence
func truncateSentence(s string, k int) string {
	for i, c := range s {
		if c == ' ' {
			k--
		}
		if k == 0 {
			return s[:i]
		}
	}
	return s
}

# Accepted solution for LeetCode #1816: Truncate Sentence
class Solution:
    def truncateSentence(self, s: str, k: int) -> str:
        return ' '.join(s.split()[:k])

// Accepted solution for LeetCode #1816: Truncate Sentence
struct Solution;

impl Solution {
    fn truncate_sentence(s: String, k: i32) -> String {
        let words: Vec<&str> = s.split_whitespace().take(k as usize).collect();
        words.join(" ")
    }
}

#[test]
fn test() {
    let s = "Hello how are you Contestant".to_string();
    let k = 4;
    let res = "Hello how are you".to_string();
    assert_eq!(Solution::truncate_sentence(s, k), res);
}

// Accepted solution for LeetCode #1816: Truncate Sentence
function truncateSentence(s: string, k: number): string {
    for (let i = 0; i < s.length; ++i) {
        if (s[i] === ' ' && --k === 0) {
            return s.slice(0, i);
        }
    }
    return s;
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.