Sentence Reverser - Problem

Given a string containing words separated by spaces, reverse the order of the words while keeping each word intact.

For example, if the input is "hello world programming", the output should be "programming world hello".

Note: Words are separated by single spaces, and there are no leading or trailing spaces in the input.

Input & Output

Example 1 — Basic Case

$ Input: sentence = "hello world code"

› Output: "code world hello"

💡 Note: The words are reversed in order: 'hello' becomes last, 'world' stays in middle, 'code' becomes first

Example 2 — Two Words

$ Input: sentence = "good morning"

› Output: "morning good"

💡 Note: Simple swap: 'good' and 'morning' switch positions

Example 3 — Single Word

$ Input: sentence = "programming"

› Output: "programming"

💡 Note: Only one word, so the result is the same as input

Constraints

1 ≤ sentence.length ≤ 10⁴
sentence contains only letters and spaces
No leading or trailing spaces
Words are separated by single spaces

Visualization

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Asked in

G Google 25 a Amazon 30 M Microsoft 20

The key insight is to use built-in string functions like split and reverse instead of manual character processing. The optimal approach splits the sentence by spaces, reverses the word array, then joins back with spaces. Time: O(n), Space: O(n)

Common Approaches

✓ Manual Character Processing

⏱️ Time: O(n) Space: O(n)

Start from the end of the string and work backwards, collecting characters to form words. When we hit a space, we know we've completed a word and add it to our result.

Split and Reverse

⏱️ Time: O(n) Space: O(n)

Split the sentence into individual words using the space delimiter, reverse the resulting array, then join the words back together with spaces.

Manual Character Processing — Algorithm Steps

Start from the last character
Build each word backwards by collecting characters
When hitting a space, add the completed word to result
Continue until reaching the beginning

Visualization

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Step-by-Step Walkthrough

Scan Right to Left

Start from end, collect characters

Build Words

When hitting space, complete current word

Join Result

Combine reversed words with spaces

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* solution(char* sentence) {
    int len = strlen(sentence);
    if (len == 0) {
        char* result = malloc(1);
        result[0] = '\0';
        return result;
    }
    
    char* result = malloc(len + 1);
    char* words[1000];
    int wordCount = 0;
    
    char currentWord[1000];
    int wordPos = 0;
    
    // Process from right to left
    for (int i = len - 1; i >= 0; i--) {
        if (sentence[i] == ' ') {
            if (wordPos > 0) {
                currentWord[wordPos] = '\0';
                words[wordCount] = malloc(strlen(currentWord) + 1);
                strcpy(words[wordCount], currentWord);
                wordCount++;
                wordPos = 0;
            }
        } else {
            // Insert at beginning
            for (int j = wordPos; j > 0; j--) {
                currentWord[j] = currentWord[j-1];
            }
            currentWord[0] = sentence[i];
            wordPos++;
        }
    }
    
    // Don't forget the last word
    if (wordPos > 0) {
        currentWord[wordPos] = '\0';
        words[wordCount] = malloc(strlen(currentWord) + 1);
        strcpy(words[wordCount], currentWord);
        wordCount++;
    }
    
    // Build final result
    result[0] = '\0';
    for (int i = 0; i < wordCount; i++) {
        if (i > 0) strcat(result, " ");
        strcat(result, words[i]);
        free(words[i]);
    }
    
    return result;
}

int main() {
    char sentence[10000];
    fgets(sentence, sizeof(sentence), stdin);
    sentence[strcspn(sentence, "\n")] = 0;
    
    char* result = solution(sentence);
    printf("%s\n", result);
    free(result);
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through the string from right to left

✓ Linear Growth

Space Complexity

O(n)

Need space to store the result string and temporary word building

⚡ Linearithmic Space

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Medium Frequency

~15 min Avg. Time

850 Likes

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Sentence Reverser - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Manual Character Processing — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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