Reverse Words in a String II - Practice Coding Problems

Reverse Words in a String II - Problem

Given a character array s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by a single space.

Your code must solve the problem in-place, i.e. without allocating extra space.

Input & Output

Example 1 — Basic Case

$ Input: s = ["t","h","e"," ","s","k","y"," ","i","s"," ","b","l","u","e"]

› Output: ["b","l","u","e"," ","i","s"," ","s","k","y"," ","t","h","e"]

💡 Note: The words are "the", "sky", "is", "blue". Reversing their order gives "blue", "is", "sky", "the".

Example 2 — Single Word

$ Input: s = ["a"]

› Output: ["a"]

💡 Note: Only one word, so it remains the same after reversal.

Example 3 — Two Words

$ Input: s = ["h","i"," ","b","y","e"]

› Output: ["b","y","e"," ","h","i"]

💡 Note: Two words: "hi" and "bye". After reversal: "bye hi".

Constraints

1 ≤ s.length ≤ 10⁴
s[i] is either a lowercase English letter or a space ' '
There is at least one word in s
s does not contain leading or trailing spaces
All words are separated by exactly one space

Visualization

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Understanding the Visualization

Input

Character array representing sentence with words separated by spaces

Process

Reverse word order while keeping individual words readable

Output

Same array with words in reversed order

Key Takeaway

🎯 Key Insight: Two reversals (entire array, then each word) achieve in-place word reordering

Asked in

M Microsoft 35 a Amazon 28 G Google 22 f Facebook 18

The key insight is using a two-step reversal: first reverse the entire array, then reverse each individual word. This achieves in-place word reversal with O(1) space. Best approach is the two-pointer technique. Time: O(n), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Two-Step Reversal (Optimal)	O(n)	O(1)	Reverse entire array, then reverse each word individually
Extra Space Approach	O(n)	O(n)	Split into words, reverse order, then join back

Two-Step Reversal (Optimal) — Algorithm Steps

Reverse the entire character array
Find each word boundary (spaces)
Reverse each word individually in-place
Result has words in reversed order

Visualization

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Step-by-Step Walkthrough

Reverse All

Reverse entire character array

Reverse Words

Reverse each individual word

Complete

Words are now in reversed order

Code -

solution.c — C

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

void reverseRange(char* s[], int start, int end) {
    while (start < end) {
        char* temp = s[start];
        s[start] = s[end];
        s[end] = temp;
        start++;
        end--;
    }
}

void solution(char* s[], int size) {
    // Step 1: Reverse the entire array
    reverseRange(s, 0, size - 1);
    
    // Step 2: Reverse each word
    int start = 0;
    for (int i = 0; i <= size; i++) {
        if (i == size || strcmp(s[i], " ") == 0) {
            // Found end of word, reverse it
            reverseRange(s, start, i - 1);
            start = i + 1;
        }
    }
}

int main() {
    // Implementation for parsing JSON input would be complex in C
    // This is a simplified version
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Two passes through array: once to reverse all, once to reverse each word

✓ Linear Growth

Space Complexity

O(1)

Only uses constant extra space for pointers and temporary variables

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Two-Step Reversal (Optimal) — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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